Ludwik Adamowicz
 Professor, Chemistry and BiochemistrySci
 Professor, Physics
 Member of the Graduate Faculty
Contact
 (520) 6216607
 Chemistry, Rm. 317
 Tucson, AZ 85721
 ludwik@email.arizona.edu
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202223 Courses

Physical Chemistry
CHEM 480B (Fall 2022) 
Statistcl Thermodynamics
CHEM 582 (Fall 2022)
202122 Courses

Dissertation
CHEM 920 (Fall 2021) 
Research
CHEM 900 (Fall 2021)
202021 Courses

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CHEM 920 (Spring 2021) 
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CHEM 900 (Spring 2021) 
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CHEM 910 (Spring 2021) 
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CHEM 920 (Fall 2020) 
Exchange Chemical Info
CHEM 695B (Fall 2020) 
Physical Chemistry
CHEM 480A (Fall 2020) 
Physical Chemistry
CHEM 480B (Fall 2020) 
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CHEM 900 (Fall 2020)
201920 Courses

Dissertation
CHEM 920 (Spring 2020) 
Research
CHEM 900 (Spring 2020) 
Dissertation
CHEM 920 (Fall 2019) 
Physical Chemistry
CHEM 480A (Fall 2019) 
Research
CHEM 900 (Fall 2019) 
Statistcl Thermodynamics
CHEM 582 (Fall 2019)
201819 Courses

Dissertation
CHEM 920 (Spring 2019) 
Physical Chemistry
CHEM 480B (Spring 2019) 
Research
CHEM 900 (Spring 2019) 
Dissertation
CHEM 920 (Fall 2018) 
Independent Study
PHYS 599 (Fall 2018) 
Statistcl Thermodynamics
CHEM 582 (Fall 2018)
201718 Courses

Dissertation
CHEM 920 (Spring 2018) 
Independent Study
PHYS 599 (Spring 2018) 
Dissertation
CHEM 920 (Fall 2017) 
Exchange Chemical Info
CHEM 695B (Fall 2017) 
Intro To Quantum Chem
CHEM 580 (Fall 2017) 
Research
CHEM 900 (Fall 2017)
201617 Courses

Dissertation
CHEM 920 (Spring 2017) 
Exchange Chemical Info
CHEM 695B (Spring 2017) 
Physical Chemistry
CHEM 480A (Spring 2017) 
Research
CHEM 900 (Spring 2017) 
Dissertation
CHEM 920 (Fall 2016) 
Exchange Chemical Info
CHEM 695B (Fall 2016) 
Physical Chemistry
CHEM 480A (Fall 2016) 
Research
CHEM 900 (Fall 2016)
201516 Courses

Dissertation
CHEM 920 (Spring 2016) 
Exchange Chemical Info
CHEM 695B (Spring 2016) 
Independent Study
PHYS 599 (Spring 2016) 
Physical Chemistry
CHEM 480A (Spring 2016) 
Research
CHEM 900 (Spring 2016)
Scholarly Contributions
Journals/Publications
 Bubin, S., & Adamowicz, L. (2014). Prediction of ^{1}P Rydberg energy levels of beryllium based on calculations with explicitly correlated Gaussians. Journal of Chemical Physics, 140(2).More infoAbstract: Benchmark variational calculations are performed for the seven lowest 1s22s np (1P), n 28, states of the beryllium atom. The calculations explicitly include the effect of finite mass of 9Be nucleus and account perturbatively for the massvelocity, Darwin, and spinspin relativistic corrections. The wave functions of the states are expanded in terms of allelectron explicitly correlated Gaussian functions. Basis sets of up to 12500 optimized Gaussians are used. The maximum discrepancy between the calculated nonrelativistic and experimental energies of 1s22s np (1P) →1s22s2 (1S) transition is about 12 cm1. The inclusion of the relativistic corrections reduces the discrepancy to bellow 0.8 cm1. © 2014 AIP Publishing LLC.
 Kirnosov, N., Sharkey, K. L., & Adamowicz, L. (2014). Erratum: Lifetimes of rovibrational levels of HD + (Physical Review A  Atomic, Molecular, and Optical Physics A (2014) 89 (012513)). Physical Review A  Atomic, Molecular, and Optical Physics, 89(2).
 Kirnosov, N., Sharkey, K. L., & Adamowicz, L. (2014). Lifetimes of rovibrational levels of HD +. Physical Review A  Atomic, Molecular, and Optical Physics, 89(1).More infoAbstract: A method for calculating the oscillator strengths for rovibrational transitions of a diatomic system within an approach that is not based on the BornOppenheimer (BO) approximation is presented. The nonBO wave functions representing the bound states of the system are expanded in terms of explicitly correlated Gaussian functions. The method is applied to calculate oscillator strengths for the HD+ ion for transitions between rotationless vibrational states and vibrational states which are rotationally singly excited. The effect of the asymmetry of the HD+ charge distribution on the oscillator strengths and on the lifetimes of the states is elucidated. © 2014 American Physical Society.
 Kirnosov, N., Sharkey, K., & Adamowicz, L. (2014). Charge asymmetry in the rovibrationally excited HD molecule. Journal of Chemical Physics, 140(10).More infoAbstract: The recently developed method for performing allparticle nonBornOppenheimer variational calculations on diatomic molecular systems excited to the first excited rotational state and simultaneously vibrationally excited is employed to study the charge asymmetry and the level lifetimes of the HD molecule. The method uses allparticle explicitly correlated Gaussian functions. The nonlinear parameters of the Gaussians are optimized with the aid of the analytical energy gradient determined with respect to these parameters. © 2014 AIP Publishing LLC.
 Slanina, Z., Uhlík, F., Lee, S., Adamowicz, L., Feng, L., Ishitsuka, M. O., Tsuchiya, T., Zhao, X., & Nagase, S. (2014). Sm@C_{74}: Computed relative isomeric populations. Fullerenes Nanotubes and Carbon Nanostructures, 22(13), 235242.More infoAbstract: Densityfunctional theory (DFT) calculations are reported for Sm@C 74, namely for Sm encapsulation in the IPR (isolated pentagon rule) C74 cage and in two C74 cages with a pentagonpentagon junction. Their relative thermodynamic production yields are evaluated using the DFT calculated terms and it is shown that the IPRbased Sm@C74 endohedral prevails at relevant temperatures in agreement with observation, however, isolation of a minor isomer could be possible. © 2014 Taylor and Francis Group, LLC.
 Slanina, Z., Uhlík, F., Lee, S., Adamowicz, L., Kurihara, H., Nikawa, H., Xing, L. u., Yamada, M., Nagase, S., & Akasaka, T. (2014). Computations on metallofullerenes derivatized during extraction: La@C _{80}C_{6}H_{3}Cl_{2} and La@C _{82}C_{6}H_{3}Cl_{2}. Fullerenes Nanotubes and Carbon Nanostructures, 22(13), 173181.More infoAbstract: Recently, an extraction of la metallofullerenes from soot using 1,2,4trichlorobenzene has been reported for La@C80 and La@C 82. In both cases, the cages were derivatized by the solvent (forming La@C80C6H3Cl2 and La@C 82C6H3Cl2) and the following Xray analysis disclosed rather unexpected cages: C80(C2v;3) and C82(C3v;7). In order to explain the challenging observations, a twostep computational treatment is presented. The first step deals with the hightemperature gasphase formation of the underivatized endohedrals while the second step models the reaction with the solvent. The Gibbs free energies were evaluated for representative temperatures and the computational scheme was able to confirm high relative populations for the observed derivatized cages. © 2014 Taylor and Francis Group, LLC.
 Slanina, Z., Uhlík, F., Lee, S., Wang, B., Adamowicz, L., Suzuki, M., Haranaka, M., Feng, L., Xing, L. u., Nagase, S., & Akasaka, T. (2014). Towards relative populations of nonisomeric metallofullerenes: La@C _{76}() vs. La_{2}@C_{76}(,17490). Fullerenes Nanotubes and Carbon Nanostructures, 22(13), 299306.More infoAbstract: Recently, a new member of the lanthanumcontaining metallofullerene family has been isolated and its structure determined from Xray crystallographic analysis, namely La @C (,17490), even with two pairs of connected pentagons, hence violating the isolatedpentagon rule (IPR). In order to explain the interesting observation, a computational treatment is suggested for evaluations of the relative populations of nonisomeric metallofullerenes. As a comparative reference structure, the most stable La@C isomer is selectedthe IPRC ()cage based endohedral La@C (). In the relevant temperature region, the La @C (,17490) species should be considerably more populated compared to La@C (). The new computational stability treatment is applicable to a wider spectrum of nonisomeric metallofullerenes. © 2014 Taylor and Francis Group, LLC.
 Stanke, M., & Adamowicz, L. (2014). Charge asymmetry and relativistic corrections in pure vibrational states of the HD^{+} ion. Physical Review A  Atomic, Molecular, and Optical Physics, 89(3).More infoAbstract: In this work we present very accurate quantummechanical calculations of all bound pure vibrational states of the HD+ ion performed without the BornOppenheimer (BO) approximation. All three particles forming the system are treated on equal footing. The approach involves separating the centerofmass motion from the laboratoryframe nonrelativistic Hamiltonian and expending the wave function of each considered state in terms of allparticle explicitly correlated Gaussian functions. The Gaussian exponential parameters are variationally optimized with the aid of the analytical energy gradient calculated with respect to these parameters. For each state the leading relativistic corrections are calculated as expectation values of the corresponding operators with the nonBO wave function of the state. The nonBO approach allows us to directly describe the charge asymmetry in HD+ which is due to the nuclearmass asymmetry. The effect increases with the vibrational excitation and affects the values of the relativistic corrections. This phenomenon is the focus of the present study. © 2014 American Physical Society.
 Tung, W., & Adamowicz, L. (2014). Accurate potential energy curve of the LiH^{+} molecule calculated with explicitly correlated Gaussian functions. Journal of Chemical Physics, 140(12).More infoAbstract: Very accurate calculations of the groundstate potential energy curve (PEC) of the LiH+ ion performed with allelectron explicitly correlated Gaussian functions with shifted centers are presented. The variational method is employed. The calculations involve optimization of nonlinear exponential parameters of the Gaussians performed with the aid of the analytical first derivatives of the energy determined with respect to the parameters. The diagonal adiabatic correction is also calculated for each PEC point. The PEC is then used to calculate the vibrational energies of the system. In that calculation, the nonadiabatic effects are accounted for by using an effective vibrational mass obtained by the minimization of the difference between the vibrational energies obtained from the calculations where the BornOppenheimer approximation was not assumed and the results of the present calculations. © 2014 AIP Publishing LLC.
 Adamowicz, L., Sharkey, K. L., Kirnosov, N., & Adamowicz, L. . (2013). An algorithm for quantum mechanical finitenuclearmass variational calculations of atoms with L = 3 using allelectron explicitly correlated Gaussian basis functions. The Journal of chemical physics, 138(10).More infoA new algorithm for quantummechanical nonrelativistic calculation of the Hamiltonian matrix elements with allelectron explicitly correlated Gaussian functions for atoms with an arbitrary number of s electrons and with three p electrons, or one p electron and one d electron, or one f electron is developed and implemented. In particular the implementation concerns atomic states with L = 3 and M = 0. The Hamiltonian used in the approach is obtained by rigorously separating the centerofmass motion from the laboratoryframe all particle Hamiltonian, and thus it explicitly depends on the finite mass of the nucleus. The approach is employed to perform test calculations on the lowest (2)F state of the two main isotopes of the lithium atom, (7)Li and (6)Li.
 Adamowicz, L., Tung, W., Pavanello, M., Sharkey, K. L., Kirnosov, N., & Adamowicz, L. . (2013). Analytical energy gradient used in variational BornOppenheimer calculations with allelectron explicitly correlated Gaussian functions for molecules containing one π electron. The Journal of chemical physics, 138(12).More infoAn algorithm for variational calculations of molecules with one π electron performed with allelectron explicitly correlated gaussian (ECG) functions with floating centers is derived and implemented. The algorithm includes the analytic gradient of the BornOppenheimer electronic energy determined with respect to the ECG exponential parameters and the coordinates of the gaussian centers. The availability of the gradient greatly accelerates the variational energy minimization. The algorithm is tested in calculations of four electronic excited states, c(3)Π(u), C(1)Π(u), i(3)Π(g), and I(1)Π(g), of the hydrogen molecule at a single internuclear distance specific to each state. With the use of the analytical energy gradient, the present calculations yield new, lowesttodate, variational energy upper bounds for all four states.
 Bubin, S., & Adamowicz, L. (2013). Prediction of 2S Rydberg energy levels of 6Li and 7Li based on quantummechanical calculations performed with explicitly correlated Gaussian functions. Physical Review A  Atomic, Molecular, and Optical Physics, 87(4).More infoAbstract: Accurate variational quantummechanical calculations are performed for the nine lowest 2S (1s2ns), n=2, ̄,10 states of the lithium atom. The effect of the finite nuclear mass is explicitly included in the calculations allowing for the determination of the isotopic shifts of the energy levels. The wave functions of the states are expanded in terms of allelectron explicitly correlated Gaussian functions and their exponential parameters are variationally optimized with the aid of the analytical energy gradient determined with respect to those parameters. The experimental results for the lower states (n=3, ̄,6) and the calculated results for the higher states (n=7, ̄,10) fitted with quantumdefectlike formulas are used to predict the energies of 2S 1s2ns states for 7Li and 6Li with n up to 30. © 2013 American Physical Society.
 Bubin, S., Pavanello, M., Tung, W., Sharkey, K. L., & Adamowicz, L. (2013). Bornoppenheimer and nonbornoppenheimer, atomic and molecular calculations with explicitly correlated gaussians. Chemical Reviews, 113(1), 3679.More infoPMID: 23020161;Abstract: The current state of efforts to use explicitly correlated Gaussian functions (ECGs) in calculations of small atomic and molecular systems with very high accuracy is studied. Such high accuracy can be achieved either by performing very precise BornOppenheimer (BO) calculations and correcting the results for adiabatic and nonadiabatic effects or by treating the nuclei and electrons on equal footing and explicitly including their motions in the Hamiltonian and in the wave function. It is shown that by using large basis sets of ECGs and by variationally optimizing their nonlinear parameters with a method based on the analytical energy gradient it is possible to determine the energies of ground and excited states of these systems with an accuracy approaching the accuracy of the most precise experimental measurements. In moving forward with the development of ECG techniques for very accurate BO and nonBO atomic and molecular calculations, several directions need to be considered.
 Bubin, S., Sharkey, K. L., & Adamowicz, L. (2013). Prediction of ^{2}D Rydberg energy levels of ^{6}Li and ^{7}Li based on very accurate quantum mechanical calculations performed with explicitly correlated Gaussian functions. Journal of Chemical Physics, 138(16).More infoPMID: 23635139;Abstract: Very accurate variational nonrelativistic finitenuclearmass calculations employing allelectron explicitly correlated Gaussian basis functions are carried out for six Rydberg 2D states (1s2nd, n 6, ⋯, 11) of the 7Li and 6Li isotopes. The exponential parameters of the Gaussian functions are optimized using the variational method with the aid of the analytical energy gradient determined with respect to these parameters. The experimental results for the lower states (n 3,⋯, 6) and the calculated results for the higher states (n 7, ⋯, 11) fitted with quantumdefectlike formulas are used to predict the energies of 2D 1s2nd states for 7Li and 6Li with n up to 30. © 2013 AIP Publishing LLC.
 Diniz, L. G., Mohallem, J. R., Alijah, A., Pavanello, M., Adamowicz, L., Polyansky, O. L., & Tennyson, J. (2013). Vibrationally and rotationally nonadiabatic calculations on H _{3}^{+} using coordinatedependent vibrational and rotational masses. Physical Review A  Atomic, Molecular, and Optical Physics, 88(3).More infoAbstract: Using the coremass approach, we have generated a vibrationalmass surface for the triatomic H3+. The coordinatedependent masses account for the offresonance nonadiabatic coupling and permit a very accurate determination of the rovibrational states using a single potential energy surface. The new, highprecision measurements of 12 rovibrational transitions in the ν2 bending fundamental of H3+ by Wu are used to scale this surface empirically and to derive statedependent vibrational and rotational masses that reproduce the experimental transition energies to 103cm1. Rotational term values for J≤10 are presented for the two lowest vibrational states and equivalent transitions in D3+ considered. © 2013 American Physical Society.
 Ivanov, V. V., Zakharov, A. B., & Adamowicz, L. (2013). Molecular dipole static polarisabilities and hyperpolarisabilities of conjugated oligomer chains calculated with the local πelectron coupled cluster theory. Molecular Physics, 111(24), 37793792.More infoAbstract: A new semiempirical πelectron local coupled cluster theory has been developed to calculate static dipole polarisabilities and hyperpolarisabilities of extended πconjugated systems. The key idea of the approach is the use of the ethylene molecular orbitals as the orbital basis set for πconjugated compounds (the method is termed the Covalent Unbonded Molecules of Ethylene method, cue). Test calculations of some small model organic conjugated compounds demonstrate high accuracy of the version of the cue local coupled cluster theory developed in this work in comparison with the πelectron full configuration interaction (FCI) method. Calculations of different conjugated carbonbased oligomer chains (polyenes, polyynes, polyacenes, polybenzocyclobutadiene, etc.) demonstrate fast convergence (per πelectron) of the polarisability and hyperpolarisability values in the calculations when more classes of orbital excitations are included in the coupled cluster single and double (CCSD) excitation operator. The results show qualitatively correct dependence on the system size. © 2013 Taylor and Francis Group, LLC.
 Kirnosov, N., Sharkey, K. L., & Adamowicz, L. (2013). Charge asymmetry in rovibrationally excited HD^{+} determined using explicitly correlated allparticle Gaussian functions. Journal of Chemical Physics, 139(20).More infoPMID: 24289342;Abstract: Very accurate nonBornOppenheimer quantummechanical calculations are performed to determine the average values of the interparticle distances and the protondeuteron density function for the rovibrationally excited HD+ ion. The states corresponding to excitations to all bound vibrational states (v = 0,., 22) and simultaneously excited to the first excited rotational state (N = 1) are considered. To describe each state up to 8000 explicitly correlated allparticle Gaussian functions are used. The nonlinear parameters of the Gaussians are variationally optimized using a procedure that employs the analytical energy gradient determined with respect to these parameters. The results show an increasing asymmetry in the electron distribution with the vibrational excitation as the electron density shifts towards deuteron and away from the proton. © 2013 AIP Publishing LLC.
 Mitroy, J., Bubin, S., Horiuchi, W., Suzuki, Y., Adamowicz, L., Cencek, W., Szalewicz, K., Komasa, J., Blume, D., & Varga, K. (2013). Theory and application of explicitly correlated Gaussians. Reviews of Modern Physics, 85(2), 693749.More infoAbstract: The variational method complemented with the use of explicitly correlated Gaussian basis functions is one of the most powerful approaches currently used for calculating the properties of fewbody systems. Despite its conceptual simplicity, the method offers great flexibility, high accuracy, and can be used to study diverse quantum systems, ranging from small atoms and molecules to light nuclei, hadrons, quantum dots, and Efimov systems. The basic theoretical foundations are discussed, recent advances in the applications of explicitly correlated Gaussians in physics and chemistry are reviewed, and the strengths and weaknesses of the explicitly correlated Gaussians approach are compared with other fewbody techniques. © 2013 American Physical Society.
 Sharkey, K. L., Kirnosov, N., & Adamowicz, L. (2013). An algorithm for nonBornOppenheimer quantum mechanical variational calculations of N = 1 rotationally excited states of diatomic molecules using allparticle explicitly correlated Gaussian functions. Journal of Chemical Physics, 139(16).More infoPMID: 24182016;Abstract: An algorithm for quantum mechanical variational calculations of bound states of diatomic molecules corresponding to the total angular momentum quantum number equal to one (N = 1) is derived and implemented. The approach employs allparticle explicitly correlated Gaussian function for the wavefunction expansion. The algorithm is tested in the calculations of the N = 1, v = 0,., 22 states of the HD+ ion. © 2013 AIP Publishing LLC.
 Sharkey, K. L., Kirnosov, N., & Adamowicz, L. (2013). An algorithm for quantum mechanical finitenuclearmass variational calculations of atoms with L = 3 using allelectron explicitly correlated Gaussian basis functions. Journal of Chemical Physics, 138(10).More infoPMID: 23514465;Abstract: A new algorithm for quantummechanical nonrelativistic calculation of the Hamiltonian matrix elements with allelectron explicitly correlated Gaussian functions for atoms with an arbitrary number of s electrons and with three p electrons, or one p electron and one d electron, or one f electron is developed and implemented. In particular the implementation concerns atomic states with L 3 and M 0. The Hamiltonian used in the approach is obtained by rigorously separating the centerofmass motion from the laboratoryframe all particle Hamiltonian, and thus it explicitly depends on the finite mass of the nucleus. The approach is employed to perform test calculations on the lowest 2F state of the two main isotopes of the lithium atom, 7Li and 6Li. © 2013 American Institute of Physics.
 Sharkey, K. L., Kirnosov, N., & Adamowicz, L. (2013). NonBornOppenheimer method for direct variational calculations of diatomic first excited rotational states using explicitly correlated allparticle Gaussian functions. Physical Review A  Atomic, Molecular, and Optical Physics, 88(3).More infoAbstract: We report the development of a direct variational method for calculating the first rotational excited state of diatomic molecules with σ electrons where the BornOppenheimer approximation is not assumed. The method employs allparticle explicitly correlated Gaussian basis functions. The exponential parameters of the Gaussians are optimized with the aid of an analytically calculated energy gradient determined with respect to these parameters. The method is tested in calculations of the orthopara spin isomerization of the hydrogen molecule in its allbound vibrational states. © 2013 American Physical Society.
 Stanke, M., & Adamowicz, L. (2013). Molecular relativistic corrections determined in the framework where the bornoppenheimer approximation is not assumed. Journal of Physical Chemistry A, 117(39), 1012910137.More infoAbstract: In this work, we describe how the energies obtained in molecular calculations performed without assuming the BornOppenheimer (BO) approximation can be augmented with corrections accounting for the leading relativistic effects. Unlike the conventional BO approach, where these effects only concern the relativistic interactions between the electrons, the nonBO approach also accounts for the relativistic effects due to the nuclei and due to the coupling of the coupled electronnucleus motion. In the numerical sections, the results obtained with the two approaches are compared. The first comparison concerns the dissociation energies of the twoelectron isotopologues of the H2 molecule, H2, HD, D2, T2, and the HeH + ion. The comparison shows that, as expected, the differences in the relativistic contributions obtained with the two approaches increase as the nuclei become lighter. The second comparison concerns the relativistic corrections to all 23 pure vibrational states of the HD+ ion. An interesting charge asymmetry caused by the nonadiabatic electronnucleus interaction appears in this system, and this effect significantly increases with the vibration excitation. The comparison of the nonBO results with the results obtained with the conventional BO approach, which in the lowest order does not describe the chargeasymmetry effect, reveals how this effect affects the values of the relativistic corrections. © 2013 American Chemical Society.
 Stanke, M., Adamowicz, L., & Kedziera, D. (2013). Selection of a Gaussian basis set for calculating the Bethe logarithm for the ground state of the hydrogen atom. Molecular Physics, 111(911), 10631068.More infoAbstract: The inclusion of the leading quantum electrodynamic energy correction is unavoidable if one aims for sub 0.01 cm1 accuracy in the calculations of atomic and molecular spectra. The important part of the calculation of the correction is the determination of the value of the socalled Bethe logarithm (ln k 0). In this work, we examine how to construct Gaussian basis sets for such calculations. While the testing is performed for the ground state of the hydrogen atom, the results allow us to suggest how the construction of appropriate basis sets can be handled in ln k 0 calculations of atoms and molecules with more than one electron performed with explicitly correlated allelectron Gaussian functions. © 2013 Copyright Taylor and Francis Group, LLC.
 Stepanian, S. G., Ivanov, A. Y., & Adamowicz, L. (2013). Conformational composition of neutral leucine. Matrix isolation infrared and ab initio study. Chemical Physics, 423, 2029.More infoAbstract: Lowtemperature matrixisolation FTIR spectroscopy and ab initio calculations are employed to determine conformational composition of neutral leucine. The presence of three leucine conformers in the matrices is revealed. This is in agreement with the results of a detailed study of the potential energy surface of leucine which demonstrates that only five out of 105 possible conformers should have populations in the matrices larger than 2% and only three conformers, which are the ones detected in the experiment, should have populations larger than 10%. UV irradiation of the matrix samples are used to separate bands of the different conformers. We also show that the populations of the leucine conformers in the gas phase at 440 K are significantly different from the ones in matrices. The population of the lowest energy con former in the gas phase being approximately 23% in the gas phase increases to over 64% in matrices. © 2013 Elsevier B.V. All rights reserved.
 Tung, W., Pavanello, M., Sharkey, K. L., Kirnosov, N., & Adamowicz, L. (2013). Analytical energy gradient used in variational BornOppenheimer calculations with allelectron explicitly correlated Gaussian functions for molecules containing one π electron. Journal of Chemical Physics, 138(12).More infoPMID: 23556703;Abstract: An algorithm for variational calculations of molecules with one π electron performed with allelectron explicitly correlated Gaussian (ECG) functions with floating centers is derived and implemented. The algorithm includes the analytic gradient of the BornOppenheimer electronic energy determined with respect to the ECG exponential parameters and the coordinates of the Gaussian centers. The availability of the gradient greatly accelerates the variational energy minimization. The algorithm is tested in calculations of four electronic excited states, c3Πu, C1Π u, i3Πg, and I1Πg, of the hydrogen molecule at a single internuclear distance specific to each state. With the use of the analytical energy gradient, the present calculations yield new, lowesttodate, variational energy upper bounds for all four states. © 2013 American Institute of Physics.
 Uhlík, F., Slanina, Z., Lee, S., Adamowicz, L., & Nagase, S. (2013). Stability calculations for Eu@C_{74} isomers. International Journal of Quantum Chemistry, 113(5), 729733.More infoAbstract: Density functional theory (DFT) calculations are reported for Eu@C 74, where the considered C74 cage is either the IPR (isolated pentagon rule) species or two cages with a pentagonpentagon junction. Their relative thermodynamic production yields are evaluated using the DFT calculated terms and it is shown that the IPRbased Eu@C74 endohedral prevails at relevant temperatures in agreement with observation, however, isolation of a minor isomer should be possible. The question of whether C 74based metallofullerenes present isomeric forms is still an object of debate. In the case of Eu@C74 density functional theory calculations predict that in addition to the dominant endohedral species (in which none of the pentagons in the C74 cage make contact with each, in respect to the isolated pentagon rule, IPR), a nonIPR isomer should exist and could possibly even be isolated. Copyright © 2012 Wiley Periodicals, Inc.
 Adamowicz, L., & Pavanello, M. (2012). Progress in calculating the potential energy surface of H _{3}^{+}. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 370(1978), 50015013.More infoPMID: 23028149;Abstract: The most accurate electronic structure calculations are performed using wave function expansions in terms of basis functions explicitly dependent on the interelectron distances. In our recent work, we use such basis functions to calculate a highly accurate potential energy surface (PES) for the H 3+ ion. The functions are explicitly correlated Gaussians, which include interelectron distances in the exponent. Key to obtaining the high accuracy in the calculations has been the use of the analytical energy gradient determined with respect to the Gaussian exponential parameters in the minimization of the RayleighRitz variational energy functional. The effective elimination of linear dependences between the basis functions and the automatic adjustment of the positions of the Gaussian centres to the changing molecular geometry of the system are the keys to the success of the computational procedure. After adiabatic and relativistic corrections are added to the PES and with an effective accounting of the nonadiabatic effects in the calculation of the rotational/vibrational states, the experimental H 3+ rovibrational spectrum is reproduced at the 0.1 cm 1 accuracy level up to 16 600 cm 1 above the ground state. © 2012 The Royal Society.
 Adamowicz, L., Sharkey, K. L., Bubin, S., & Adamowicz, L. . (2012). Explicitly correlated gaussian calculations of the 2D Rydberg states of the boron atom. The Journal of chemical physics, 137(6).More infoAccurate nonrelativistic variational calculations are performed for the seven lowest members of the (2)D Rydberg series (1s(2)2s2p(2), and 1s(2)2s(2)nd, n = 3, ..., 8) of the boron atom. The wave functions of the states are expanded in terms of allelectron explicitly correlated gaussian basis functions and the effect of the finite nuclear mass is directly included in the calculations allowing for determining the isotopic shifts of the energy levels. The gaussian basis is optimized independently for each state with the aid of the analytic energy gradient with respect to the gaussian parameters. The calculations represent the highest accuracy level currently achievable for the considered states. The computed energies are compared with the available experimental data.
 Adamowicz, L., Tung, W., Pavanello, M., & Adamowicz, L. . (2012). Accurate potential energy curves for HeH+ isotopologues. The Journal of chemical physics, 137(16).More infoNew accurate groundstate potential energy curves (PEC) for the (4)HeH(+), (4)HeD(+), (3)HeH(+), and (3)HeD(+) isotopologues are calculated with 600 explicitly correlated Gaussian (ECG) functions with shifted centers in the range between R = 0.35 a(0) and 145 a(0). The calculations include the adiabatic corrections (AC). The absolute accuracy of all BornOppenheimer (BO) PEC points is better than 0.0018 cm(1) and it is better than 0.0005 cm(1) for the ACs. With respect to the very recent BO PEC calculations performed by Pachucki with 20 000 generalized HeitlerLondon explicitly correlated functions [K. Pachucki, Phys. Rev. A 85, 042511 (2012)], the present energy calculated at R = 1.46 a(0) (a point near the BO equilibrium distance) lies above by only 0.0012 cm(1). Using Pachucki's BO energy at the equilibrium distance of R = 1.463 283 a(0), and the adiabatic corrections calculated in this work for the (4)HeH(+), (4)HeD(+), (3)HeH(+), and (3)HeD(+) isotopologues, the following values are obtained for their PEC depths: 16 448.99893 cm(1), 16 456.86246 cm(1), 16 451.50635 cm(1), and 16 459.36988 cm(1), respectively. We also calculate the rovibrational (rovib) frequencies for the four isotopologues using the BO PEC of Pachucki augmented with the present ACs. The improvements over the BO+AC PEC of Bishop and Cheung (BC) [J. Mol. Spectrosc. 75, 462 (1979)] are 1.522 cm(1) at R = 4.5 a(0) and 0.322 cm(1) at R = ∞. To partially account for the nonadiabatic effects in the rovib calculations an effective reducedmass approach is used. With that, the present (4)HeH(+) rovibrational transitions are considerably improved over the BC transitions as compared with the experimental values. Now the rovibrational transitions near the dissociation limit are as well reproduced by the present calculations as the lower transitions. For example, for the (4)HeD(+) transitions corresponding to the ν = 139 hot bands the results are off from the experimental values by less than 0.023 cm(1). This confirms high accuracy of the present PECs at larger internuclear separations.
 Adamowicz, L., Tung, W., Pavanello, M., & Adamowicz, L. . (2012). Very accurate potential energy curve of the He2+ ion. The Journal of chemical physics, 136(10).More infoA very accurate groundstate potential energy curve (PEC) of the He(2)(+) molecule is calculated with 1200 explicitly correlated Gaussian functions with shifted centers in the range between 0.9 and 100 a(0). The calculations include the adiabatic corrections determined for the (3)He(4)He(+), (3)He(2)(+), and (4)He(2)(+) isotopologues. The absolute accuracy of the PEC is better than 0.05 cm(1) and that of the adiabatic corrections is around 0.01 cm(1). The depths of the PECs augmented with the adiabatic corrections for the three isotopologues are: 19 956.708 cm(1) for (4)He(2)(+), 19 957.054 cm(1) for (3)He(4)He(+), and 19 957.401 cm(1) for (3)He(2)(+). The rovibrational energies are also determined. For (3)He(4)He(+) the computed rovibrational transitions corresponding to the ν = 10 band differ from the experiment by less than 0.005 cm(1). For the rovibrational transitions corresponding to the ν = 2322 band the difference is around 0.012 cm(1). Presently, this represents the best agreement between theory and experiment for He(2)(+).
 Bubin, S., & Adamowicz, L. (2012). Assessment of the accuracy the experimental energies of the ^{1}P ^{o} 1s ^{2}2s6p and 1s ^{2}2s7p states of ^{9}Be based on variational calculations with explicitly correlated Gaussians. Journal of Chemical Physics, 137(10).More infoPMID: 22979867;Abstract: Benchmark variational calculations are performed for the six lowest states of the 1P o 1s 22snp state series of the 9Be atom. The wave functions of the states are expanded in terms of allparticle, explicitly correlated Gaussian basis functions and the effect of the finite nuclear mass is directly included in the calculations. The exponential parameters of the Gaussians are variationally optimized using the analytical energy gradient determined with respect to those parameters. Besides providing reference nonrelativistic energies for the considered states, the calculations also allow to assess the accuracy of the experimental energies of the 1P o 1s 22s6p and 1s 22s7p states and suggest their refinement. © 2012 American Institute of Physics.
 Bubin, S., & Adamowicz, L. (2012). Explicitly correlated Gaussian calculations of the ^{2}P ^{o} Rydberg spectrum of the lithium atom. Journal of Chemical Physics, 136(13).More infoPMID: 22482550;Abstract: Accurate quantummechanical nonrelativistic variational calculations are performed for the nine lowest members of the 2P o Rydberg series (1s 2np 1, n 2, , 10) of the lithium atom. The effect of the finite nuclear mass is included in the calculations allowing for determining the isotopic shifts of the energy levels. The wave functions of the states are expanded in terms of allelectron explicitly correlated Gaussian functions. The exponential parameters of the Gaussians are variationally optimized with the aid of the analytical energy gradient determined with respect to those parameters. The calculated state energies are compared with the available experimental data. © 2012 American Institute of Physics.
 Chagovets, V. V., Kosevich, M. V., Stepanian, S. G., Boryak, O. A., Shelkovsky, V. S., Orlov, V. V., Leontiev, V. S., Pokrovskiy, V. A., Adamowicz, L., & Karachevtsev, V. A. (2012). Noncovalent interaction of methylene blue with carbon nanotubes: Theoretical and mass spectrometry characterization. Journal of Physical Chemistry C, 116(38), 2057920590.More infoAbstract: Noncovalent interaction of methylene blue dye cation (MB +) with single walled carbon nanotubes (CNT) is characterized by molecular dynamics (MD) simulation, quantum chemical calculations, and laser desorption/ionization (LDI) mass spectrometry. The MD simulation of the (MB +) nCNT (n = 110) complexes in water demonstrates that the MB + cations are adsorbed on the nanotube surface in the monomeric form. MD reveals both parallel and perpendicular orientations of the MB + tricyclic plane in relation to the long axis of CNT when placed in the water environment. The interaction energy between the components of the complex in the perpendicular conformation, as determined by quantum chemical calculations at the DFT/M052X/631++G(d,p) level of theory, explains why the bending of the MB + cation at the sulfur atom weakens the πsystem of bonds and allows for the perpendicular orientation to occur. It is also found that the adsorbed MB + induces positive electrostatic potential around the adjacent semicylindrical segment of the nanotube. The mainly monomolecular adsorption of the MB + cations at the CNT surface leads to the absence in the LDI mass spectra of (MB +) nCNT of features corresponding to products of the reduction of MB + commonly observed in the LDI mass spectra of crystalline dyes. © 2012 American Chemical Society.
 Ivanov, A. Y., Stepanian, S. G., & Adamowicz, L. (2012). Tautomeric transitions of isocytosine isolated in argon and neon matrices induced by UV irradiation. Journal of Molecular Structure, 1025, 92104.More infoAbstract: The tautomeric equilibrium of the isocytosine molecules in the low temperature Ar and Ne matrices has been investigated by the FTIR matrix isolation spectroscopy and quantumchemical calculations at the DFT, MP2, and CCSD(T) levels of theory. Broadband UV irradiation of matrix isolated isocytosine results in an increase of the abundance of the minor isocytosine tautomer, KA1. After UV irradiation the intensity of the spectral bands assigned to this tautomer increases more than 1.5 times and the bands of other minor conformers are not observed. The values of ΔH (6.6 kJ/mol) and ΔG (6.8 kJ/mol) determined based on the experimental data are close to the ΔE value (8.3 kJ/mol) calculated at the CCSD/augccpVDZ level of theory. Two Fermi resonances involving the νCO vibration of the minor tautomer KA1 (2amino4oxopyrimidine or ketoamino) and four Fermi resonances involving the ring stretching vibrations of the most abundant tautomer EA1 (2amino4hydroxypyrimidine or enolamino) are found. The performance of the different computational methods in predicting the vibrational spectra of isocytosine is evaluated. © 2012 Elsevier B.V. All rights reserved.
 Pavanello, M., Adamowicz, L., Alijah, A., Zobov, N. F., Mizus, I. I., Polyansky, O. L., Tennyson, J., Szidarovszky, T., & Császár, A. G. (2012). Calibrationquality adiabatic potential energy surfaces for H _{3}^{+} and its isotopologues. Journal of Chemical Physics, 136(18).More infoPMID: 22583283;Abstract: Calibrationquality ab initio adiabatic potential energy surfaces (PES) have been determined for all isotopologues of the molecular ion H3+. The underlying BornOppenheimer electronic structure computations used optimized explicitly correlated shifted Gaussian functions. The surfaces include diagonal BornOppenheimer corrections computed from the accurate electronic wave functions. A fit to the 41 655 ab initio points is presented which gives a standard deviation better than 0.1cm 1 when restricted to the points up to 6000cm 1 above the first dissociation asymptote. Nuclear motion calculations utilizing this PES, called GLH3P, and an exact kinetic energy operator given in orthogonal internal coordinates are presented. The rovibrational transition frequencies for H3+, H 2D, and HD2+ are compared with high resolution measurements. The most sophisticated and complete procedure employed to compute rovibrational energy levels, which makes explicit allowance for the inclusion of nonadiabatic effects, reproduces all the known rovibrational levels of the H3+ isotopologues considered to better than 0.2cm 1. This represents a significant (orderofmagnitude) improvement compared to previous studies of transitions in the visible. Careful treatment of linear geometries is important for high frequency transitions and leads to new assignments for some of the previously observed lines. Prospects for further investigations of nonadiabatic effects in the H3+ isotopologues are discussed. In short, the paper presents (a) an extremely accurate global potential energy surface of H3+ resulting from high accuracy ab initio computations and global fit, (b) very accurate nuclear motion calculations of all available experimental line data up to 16 000cm 1, and (c) results suggest that we can predict accurately the lines of H3+ towards dissociation and thus facilitate their experimental observation. © 2012 American Institute of Physics.
 Pavanello, M., Adamowicz, L., Alijah, A., Zobov, N. F., Mizus, I. I., Polyansky, O. L., Tennyson, J., Szidarovszky, T., Császár, A. G., Berg, M., Petrignani, A., & Wolf, A. (2012). Precision measurements and computations of transition energies in rotationally cold triatomic hydrogen ions up to the midvisible spectral range. Physical Review Letters, 108(2).More infoPMID: 22324677;Abstract: Firstprinciples computations and experimental measurements of transition energies are carried out for vibrational overtone lines of the triatomic hydrogen ion H3+ corresponding to floppy vibrations high above the barrier to linearity. Action spectroscopy is improved to detect extremely weak visiblelight spectral lines on cold trapped H3+ ions. A highly accurate potential surface is obtained from variational calculations using explicitly correlated Gaussian wave function expansions. After nonadiabatic corrections, the floppy H3+ vibrational spectrum is reproduced at the 0.1cm 1 level up to 16600cm 1. © 2012 American Physical Society.
 Sharkey, K. L., Bubin, S., & Adamowicz, L. (2012). Explicitly correlated Gaussian calculations of the ^{2}D Rydberg states of the boron atom. Journal of Chemical Physics, 137(6).More infoPMID: 22897279;Abstract: Accurate nonrelativistic variational calculations are performed for the seven lowest members of the 2D Rydberg series (1s 22s2p 2, and 1s 22s 2nd, n 3,⋯, 8) of the boron atom. The wave functions of the states are expanded in terms of allelectron explicitly correlated Gaussian basis functions and the effect of the finite nuclear mass is directly included in the calculations allowing for determining the isotopic shifts of the energy levels. The Gaussian basis is optimized independently for each state with the aid of the analytic energy gradient with respect to the Gaussian parameters. The calculations represent the highest accuracy level currently achievable for the considered states. The computed energies are compared with the available experimental data. © 2012 American Institute of Physics.
 Slanina, Z., Uhlík, F., Lee, S., Adamowicz, L., Akasaka, T., & Nagase, S. (2012). Stability computations for isomers of La@C _{n} (n = 72, 74, 76). Molecules, 17(11), 1314613156.More infoPMID: 23128092;Abstract: Densityfunctional theory calculations are presented for lowenergy La@C 72, La@C 74 and La@C 76 isomers with IPR (isolated pentagon rule) and nonIPR cages. The relative isomeric production yields at high temperatures are evaluated using the calculated terms, and the relationships to observations are discussed. © 2012 by the authors.
 Stepanian, S. G., Ivanov, A. Y., Smyrnova, D. A., & Adamowicz, L. (2012). UVinduced isomerization of βalanine isolated in argon matrices. Journal of Molecular Structure, 1025, 619.More infoAbstract: We have employed lowtemperature matrixisolation FTIR spectroscopy, the density functional theory and ab initio calculations at the MP2 and CCSD(T) levels of theory to determine the conformational composition of the simplest βamino acid, βalanine. UV irradiation and thermal annealing of the samples together with the FTIR spectra of deuterated βalanine were used to separate bands of different conformers. A detailed study of the potential energy surface of βalanine obtained at the MP2/augccpVDZ level of theory reveals twenty βalanine conformers, but only five of them may exist in matrices due to their sufficiently high relative stabilities and low energy barriers separating them from each other. An analysis of the FTIR spectra allows us to confirm the presence of four βalanine conformers in argon matrices with certainty. Two of them, conformers I and II, have an NH⋯O intramolecular Hbond, the third, conformer V, has an N⋯HO Hbond, and the fourth, conformer IV, has no intramolecular Hbonds. The relative populations of the conformers determined using the relative Gibbs free energies calculated at the CCSD(T)/CBS level of theory at 420 K are 48.1%, 23.7%, 16.8% and 3.2% for the conformers I, II, IV, and V, respectively. Some trace amount of conformer VII was also detected. © 2012 Elsevier B.V. All rights reserved.
 Tung, W., Pavanello, M., & Adamowicz, L. (2012). Accurate potential energy curves for HeH ^{+} isotopologues. Journal of Chemical Physics, 137(16).More infoPMID: 23126708;Abstract: New accurate groundstate potential energy curves (PEC) for the 4HeH +, 4HeD +, 3HeH +, and 3HeD + isotopologues are calculated with 600 explicitly correlated Gaussian (ECG) functions with shifted centers in the range between R = 0.35 a 0 and 145 a 0. The calculations include the adiabatic corrections (AC). The absolute accuracy of all BornOppenheimer (BO) PEC points is better than 0.0018 cm 1 and it is better than 0.0005 cm 1 for the ACs. With respect to the very recent BO PEC calculations performed by Pachucki with 20 000 generalized HeitlerLondon explicitly correlated functions [K. Pachucki, Phys. Rev. A 85, 042511 (2012)]10.1103/PhysRevA.85.042511, the present energy calculated at R = 1.46 a 0 (a point near the BO equilibrium distance) lies above by only 0.0012 cm 1. Using Pachuckis BO energy at the equilibrium distance of R = 1.463 283 a 0, and the adiabatic corrections calculated in this work for the 4HeH +, 4HeD +, 3HeH +, and 3HeD + isotopologues, the following values are obtained for their PEC depths: 16 448.99893 cm 1, 16 456.86246 cm 1, 16 451.50635 cm 1, and 16 459.36988 cm 1, respectively. We also calculate the rovibrational (rovib) frequencies for the four isotopologues using the BO PEC of Pachucki augmented with the present ACs. The improvements over the BO+AC PEC of Bishop and Cheung (BC) [J. Mol. Spectrosc. 75, 462 (1979)]10.1016/00222852(79)900900 are 1.522 cm 1 at R = 4.5 a 0 and 0.322 cm 1 at R . To partially account for the nonadiabatic effects in the rovib calculations an effective reducedmass approach is used. With that, the present 4HeH + rovibrational transitions are considerably improved over the BC transitions as compared with the experimental values. Now the rovibrational transitions near the dissociation limit are as well reproduced by the present calculations as the lower transitions. For example, for the 4HeD + transitions corresponding to the ν = 139 hot bands the results are off from the experimental values by less than 0.023 cm 1. This confirms high accuracy of the present PECs at larger internuclear separations. © 2012 American Institute of Physics.
 Tung, W., Pavanello, M., & Adamowicz, L. (2012). Very accurate potential energy curve of the He _{2}^{+} ion. Journal of Chemical Physics, 136(10).More infoPMID: 22423840;Abstract: A very accurate groundstate potential energy curve (PEC) of the He 2+ molecule is calculated with 1200 explicitly correlated Gaussian functions with shifted centers in the range between 0.9 and 100 a 0. The calculations include the adiabatic corrections determined for the 3He 4He +, 3 He 2+, and 4 He 2+ isotopologues. The absolute accuracy of the PEC is better than 0.05cm 1 and that of the adiabatic corrections is around 0.01cm 1. The depths of the PECs augmented with the adiabatic corrections for the three isotopologues are: 19956.708cm 1 for 4 He 2+, 19957.054cm 1 for 3He 4He +, and 19957.401cm 1 for 3 He 2+. The rovibrational energies are also determined. For 3He 4He + the computed rovibrational transitions corresponding to the = 10 band differ from the experiment by less than 0.005cm 1. For the rovibrational transitions corresponding to the = 2322 band the difference is around 0.012cm 1. Presently, this represents the best agreement between theory and experiment for He 2+. © 2012 American Institute of Physics.
 Adamowicz, L., Sharkey, K. L., & Adamowicz, L. . (2011). Exponentially and preexponentially correlated Gaussians for atomic quantum calculations. The Journal of chemical physics, 134(9).More infoExplicitly correlated, nelectron, onecenter s Gaussian (ECG) functions that depend on the interelectron distances in the exponent are combined with s ECGs which also depend on the interelectron separations through preexponential r(ij)(2) multipliers. The preexponentially r(ij)(2)dependent ECGs are included in the basis to better describe the interelectron correlation and the interelectron cusps. The basis set is tested in the calculations of the ground state of the beryllium atom ((9)Be).
 Adamowicz, L., Sharkey, K. L., Bubin, S., & Adamowicz, L. . (2011). An algorithm for calculating atomic D states with explicitly correlated gaussian functions. The Journal of chemical physics, 134(4).More infoAn algorithm for the variational calculation of atomic D states employing nelectron explicitly correlated gaussians is developed and implemented. The algorithm includes formulas for the first derivatives of the hamiltonian and overlap matrix elements determined with respect to the gaussian nonlinear exponential parameters. The derivatives are used to form the energy gradient which is employed in the variational energy minimization. The algorithm is tested in the calculations of the two lowest D states of the lithium and beryllium atoms. For the lowest D state of Li the present result is lower than the best previously reported result.
 Adamowicz, L., Sharkey, K. L., Bubin, S., & Adamowicz, L. . (2011). Refinement of the experimental energy levels of higher 2D Rydberg states of the lithium atom with very accurate quantum mechanical calculations. The Journal of chemical physics, 134(19).More infoVery accurate variational nonrelativistic calculations are performed for four higher Rydberg (2)D states (1s(2)nd(1), n = 8,..., 11) of the lithium atom ((7)Li). The wave functions of the states are expanded in terms of allelectron explicitly correlated Gaussian functions and finite nuclear mass is used. The exponential parameters of the Gaussians are optimized using the variational method with the aid of the analytical energy gradient determined with respect to those parameters. The results of the calculations allow for refining the experimental energy levels determined with respect to the (2)S 1s(2)2s(1) ground state.
 Adamowicz, L., Tung, W., Pavanello, M., & Adamowicz, L. . (2011). Very accurate potential energy curve of the LiH molecule. The Journal of chemical physics, 134(6).More infoWe present very accurate calculations of the groundstate potential energy curve (PEC) of the LiH molecule performed with allelectron explicitly correlated Gaussian functions with shifted centers. The PEC is generated with the variational method involving simultaneous optimization of all Gaussians with an approach employing the analytical first derivatives of the energy with respect to the Gaussian nonlinear parameters (i.e., the exponents and the coordinates of the shifts). The LiH internuclear distance is varied between 1.8 and 40 bohrs. The absolute accuracy of the generated PEC is estimated as not exceeding 0.3 cm(1). The adiabatic corrections for the four LiH isotopologues, i.e., (7)LiH, (6)LiH, (7)LiD, and (6)LiD, are also calculated and added to the LiH PEC. The aforementioned PECs are then used to calculate the vibrational energies for these systems. The maximum difference between the computed and the experimental vibrational transitions is smaller than 0.9 cm(1). The contribution of the adiabatic correction to the dissociation energy of (7)LiH molecule is 10.7 cm(1). The magnitude of this correction shows its importance in calculating the LiH spectroscopic constants. As the estimated contribution of the nonadiabatic and relativistic effects to the ground state dissociation energy is around 0.3 cm(1), their inclusion in the LiH PEC calculation seems to be the next most important contribution to evaluate in order to improve the accuracy achieved in this work.
 Bubin, S., & Adamowicz, L. (2011). Accurate variational calculations of the ground ^{2}P ^{o}(1s ^{2}2s ^{2}2p) and excited ^{2}S(1s ^{2}2s2p ^{2}) and ^{2}P ^{o}(1s ^{2}2s ^{2}3p) states of singly ionized carbon atom. Journal of Chemical Physics, 135(21).More infoPMID: 22149776;Abstract: In this article we report accurate nonrelativistic variational calculations of the ground and two excited states of C + ion. We employ extended and well optimized basis sets of allelectron explicitly correlated Gaussians to represent the wave functions of the states. The optimization of the basis functions is performed with a procedure employing the analytic gradient of the energy with respect to the nonlinear parameters of the Gaussians. The calculations explicitly include the effects due to the finite nuclear mass. The calculated transition energies between the three states are compared to the experimentally derived values. Finally, we present expectation values of some small positive and negative powers of the interparticle distances and contact densities. © 2011 American Institute of Physics.
 Bubin, S., & Adamowicz, L. (2011). CorrelatedGaussian calculations of the ground and lowlying excited states of the boron atom. Physical Review A  Atomic, Molecular, and Optical Physics, 83(2).More infoAbstract: Benchmark variational calculations of the four lowest 2P and 2S states of the boron atom (including the ground state) have been performed. The wave functions of the states have been expanded in terms of allparticle explicitly correlated Gaussian basis functions and the finite mass of the nucleus has been explicitly accounted for. Variational upper bounds for the nonrelativistic finite and infinitenuclearmass energies of all considered states have been obtained with the relative convergence of the order of 10710 8. Expectation values of the powers of the interparticle distances and Dirac δ functions depending on those distances have also been computed. These calculations provide reference values that can be used to test other highlevel quantum chemistry methods. © 2011 American Physical Society.
 Bubin, S., Stanke, M., & Adamowicz, L. (2011). Accurate nonBornOppenheimer calculations of the complete pure vibrational spectrum of D_{2} with including relativistic corrections. Journal of Chemical Physics, 135(7).More infoPMID: 21861559;Abstract: In this work we report very accurate variational calculations of the complete pure vibrational spectrum of the D2 molecule performed within the framework where the BornOppenheimer (BO) approximation is not assumed. After the elimination of the centerofmass motion, D2 becomes a threeparticle problem in this framework. As the considered states correspond to the zero total angular momentum, their wave functions are expanded in terms of allparticle, onecenter, spherically symmetric explicitly correlated Gaussian functions multiplied by even nonnegative powers of the internuclear distance. The nonrelativistic energies of the states obtained in the nonBO calculations are corrected for the relativistic effects of the order of 2 (where 1c is the fine structure constant) calculated as expectation values of the operators representing these effects. © 2011 American Institute of Physics.
 Bubin, S., Stanke, M., & Adamowicz, L. (2011). Complete pure vibrational spectrum of HD calculated without the BornOppenheimer approximation and including relativistic corrections. Physical Review A  Atomic, Molecular, and Optical Physics, 83(4).More infoAbstract: All 18 bound pure vibrational levels of the HD molecule have been calculated within the framework that does not assume the BornOppenheimer (BO) approximation. The nonrelativistic energies of the states have been corrected for the relativistic effects of the order of α2 (where α is the fine structure constant), calculated using the perturbation theory with the nonrelativistic nonBO wave functions being the zeroorder approximation. The calculations were performed by expanding the nonBO wave functions in terms of onecenter explicitly correlated Gaussian functions multiplied by even powers of the internuclear distance and by performing extensive optimization of the Gaussian nonlinear parameters. Up to 10 000 basis functions were used for each state. © 2011 American Physical Society.
 Bubin, S., Stanke, M., & Adamowicz, L. (2011). Vibrational transitions of the ^{7}LiH^{+} ion calculated without the BornOppenheimer approximation and with leading relativistic corrections. Journal of Chemical Physics, 134(2).More infoPMID: 21241076;Abstract: We recently presented very accurate calculations of the fundamental vibrational frequency of the 7LiH+ and 3He 4He+ ions [Stanke et al., Phys. Rev. A 79, 060501(R) (2009)] performed without the BornOppenheimer approximation and included leading relativistic corrections. The accuracy of those calculations was estimated to be of the order of 0.06 cm1. In the present work we extend the calculations to the remaining pure vibrational states of 7LiH+ and similarly accurate results are generated. They may lead to the experimental search for still unidentified lines corresponding to those transitions. © 2011 American Institute of Physics.
 Ivanov, V. V., Lyakh, D. I., & Adamowicz, L. (2011). Statespecific multireference coupledcluster theory of molecular electronic excited states. Annual Reports on the Progress of Chemistry  Section C, 107, 169198.More infoAbstract: A statespecific coupled cluster (CC) theory with the CAS (complete active space) reference (CASCC) and based on an approach that employs single reference determinant (socalled "formal reference" determinant) in the expression for the wave function has been developed to study electronic excited states with different spatial and spin symmetries. The formal reference determinant is used to generate the reference wave function in the form of a linear combination of the CAS determinants contracted to configurations with the spin and spatial symmetries of the target state. Such properly symmetrized multideterminantal reference provides the zeroorder description of the state under consideration. To that reference an exponential CC operator is applied to describe the dynamic electron correlation effects in the CASCC wave function of the considered state. All necessary equations for the CASCC energy and configurational amplitudes are generated using an automated computerbased method which constructs all necessary coupled cluster diagrams for any arbitrary level of the electronic excitation. © 2011 The Royal Society of Chemistry.
 Karachevtsev, V. A., Stepanian, S. G., Glamazda, A. Y., Karachevtsev, M. V., Eremenko, V. V., Lytvyn, O. S., & Adamowicz, L. (2011). Noncovalent interaction of singlewalled carbon nanotubes with 1pyrenebutanoic acid succinimide ester and glucoseoxidase. Journal of Physical Chemistry C, 115(43), 2107221082.More infoAbstract: Peculiarities of the interface interactions of 1pyrenebutanoic acid Nhydroxysuccinimide ester (PSE) with singlewalled carbon nanotubes (SWCNTs) and enzyme glucoseoxidase (GOX) have been studied with the resonance Raman spectroscopy and theoretical calculations employing the DFT method and the molecular dynamics (MD) simulation. The interaction of a nanotube with PSE leads to a downshift of the band assigned in the Raman spectrum to the tangential mode of the hybrid with respect to the position of this mode in the spectrum of the pristine SWCNT. The MD simulation demonstrates that the direct interaction between SWCNT and GOX is very strong. This interaction can be expected to change the structure of the enzyme and to significantly affect its activity. The MD simulation also shows that only one PSE molecule used as a linker between SWCNT and GOX is enough to keep GOX near the nanotube surface in the water surrounding and to prevent strong interaction between SWCNT and GOX. However, to stabilize this nanobiohybrid in water at least two PSE linkers are needed. The molecular structure of PSE is determined using the density functional theory approach (DFT/B3LYP/631++G(d,p). The geometries and the relative stabilities of all possible PSE conformers are characterized in the calculations. High structural flexibility of the PSE molecule is demonstrated. Calculations (at the M052X level of theory) have also been performed on the structures and the interaction energies of complexes formed by various SWCNTs with PSE and pyrene. Pyrene interacts strongly with the surface of carbon nanotubes with different chiralities, but the interaction with zigzag nanotubes is stronger than with armchair ones of the same diameter. Increasing the diameter of the SWCNTs leads to a higher adsorption energy, reaching the maximum value for graphene (20.8 kcal/mol). © 2011 American Chemical Society.
 Sharkey, K. L., & Adamowicz, L. (2011). Exponentially and preexponentially correlated Gaussians for atomic quantum calculations. Journal of Chemical Physics, 134(9).More infoPMID: 21384947;Abstract: Explicitly correlated, nelectron, onecenter s Gaussian (ECG) functions that depend on the interelectron distances in the exponent are combined with s ECGs which also depend on the interelectron separations through preexponential rij2 multipliers. The preexponentially r ij2dependent ECGs are included in the basis to better describe the interelectron correlation and the interelectron cusps. The basis set is tested in the calculations of the ground state of the beryllium atom (9Be). © 2011 American Institute of Physics.
 Sharkey, K. L., Bubin, S., & Adamowicz, L. (2011). 1D states of the beryllium atom: Quantum mechanical nonrelativistic calculations employing explicitly correlated Gaussian functions. Physical Review A  Atomic, Molecular, and Optical Physics, 84(4).More infoAbstract: Very accurate finitenuclearmass variational nonrelativistic calculations are performed for the lowest five 1D states (1s22p2, 1s22s13d1, 1s22s14d1, 1s22s15d1, and 1s22s16d1) of the beryllium atom (9Be). The wave functions of the states are expanded in terms of allelectron explicitly correlated Gaussian functions. The exponential parameters of the Gaussians are optimized using the variational method with the aid of the analytical energy gradient determined with respect to those parameters. The calculations exemplify the level of accuracy that is now possible with Gaussians in describing bound states of a fourelectron system where some of the electrons are excited into higher angular states. © 2011 American Physical Society.
 Sharkey, K. L., Bubin, S., & Adamowicz, L. (2011). An algorithm for calculating atomic D states with explicitly correlated Gaussian functions. Journal of Chemical Physics, 134(4).More infoPMID: 21280700;Abstract: An algorithm for the variational calculation of atomic D states employing nelectron explicitly correlated Gaussians is developed and implemented. The algorithm includes formulas for the first derivatives of the Hamiltonian and overlap matrix elements determined with respect to the Gaussian nonlinear exponential parameters. The derivatives are used to form the energy gradient which is employed in the variational energy minimization. The algorithm is tested in the calculations of the two lowest D states of the lithium and beryllium atoms. For the lowest D state of Li the present result is lower than the best previously reported result. © 2011 American Institute of Physics.
 Sharkey, K. L., Bubin, S., & Adamowicz, L. (2011). Lower Rydberg 2D states of the lithium atom: Finitenuclearmass calculations with explicitly correlated Gaussian functions. Physical Review A  Atomic, Molecular, and Optical Physics, 83(1).More infoAbstract: Very accurate variational nonrelativistic calculations are performed for the five lowest Rydberg 2D states (1s2nd1, n=3,...,7) of the lithium atom (Li7). The finitenuclearmass approach is employed and the wave functions of the states are expanded in terms of allelectron explicitly correlated Gaussian function. Four thousand Gaussians are used for each state. The calculated relative energies of the states determined with respect to the 2S 1s22s1 ground state are systematically lower than the experimental values by about 2.5 cm⊃1. As this value is about the same as the difference between the experimental relative energy between Li7⊃+ and Li7 in their groundstate energy and the corresponding calculated nonrelativistic relative energy, we attribute it to the relativistic effects not included in the present calculations. © 2011 The American Physical Society.
 Sharkey, K. L., Bubin, S., & Adamowicz, L. (2011). Refinement of the experimental energy levels of higher ^{2}D Rydberg states of the lithium atom with very accurate quantum mechanical calculations. Journal of Chemical Physics, 134(19).More infoPMID: 21599051;Abstract: Very accurate variational nonrelativistic calculations are performed for four higher Rydberg 2D states (1s2nd1, n 8, , 11) of the lithium atom (7Li). The wave functions of the states are expanded in terms of allelectron explicitly correlated Gaussian functions and finite nuclear mass is used. The exponential parameters of the Gaussians are optimized using the variational method with the aid of the analytical energy gradient determined with respect to those parameters. The results of the calculations allow for refining the experimental energy levels determined with respect to the 2S 1s22s1 ground state. © 2011 American Institute of Physics.
 Slanina, Z., Uhlík, F., Lee, S., Adamowicz, L., Akasaka, T., & Nagase, S. (2011). Calculations of metallofullerene yields. Journal of Computational and Theoretical Nanoscience, 8(11), 22332239.More infoAbstract: The paper reports computations for Mg@C 74, Ca@C 74, Sr@C 74, Ba@C 74, and all lanthanoids, based on encapsulation into the only C 74 IPR (isolated pentagon rule) cage, and for Al@C 82, Sc@C 82, Y@C 82 and La@C 82 based on encapsulation into the IPR C 2v C 82 cage. Their structural and energetic characteristics are used for evaluations of the relative production yields, employing the encapsulation Gibbsenergy terms and saturated metal pressures. It is moreover shown that the encapsulation potentialenergy changes in such series can be well related to the mere ionization potentials of the free metal atoms. Copyright © 2011 American Scientific Publishers.
 Slanina, Z., Uhlík, F., Lee, S., Adamowicz, L., Akasaka, T., & Nagase, S. (2011). Computed stabilities in metallofullerene series: Al@C_{82}, Sc@C_{82}, Y@C_{82}, and La@C_{82}. International Journal of Quantum Chemistry, 111(11), 27122718.More infoAbstract: This article reports computations for Al@C82, Sc@C82, Y@C82, and La@C82 based on encapsulation into the IPR (isolated pentagon rule) C2v C82 cage. Their structural and bonding features are also used for evaluations of the relative production yields, employing the encapsulation Gibbsenergy terms and saturated metal pressures. The results can be well related to the ionization potentials of the free metal atoms and thus also rationalized. Copyright © 2010 Wiley Periodicals, Inc.
 Slanina, Z., Uhlík, F., Lee, S., Mizorogi, N., Akasaka, T., & Adamowicz, L. (2011). Calculated relative yields for Sc_{2}S@C_{82} and Y_{2}S@C_{82}. Theoretical Chemistry Accounts, 130(23), 549554.More infoAbstract: The paper reports computations for a newly observed class of the mixed, sulfurcontaining X2S@C82 metallofullerenes, namely for X = Sc and Y, based on encapsulation into the Cs and C3v C82 isolated pentagon rule cages. Their structural, vibrational, and energetic characteristics from the densityfunctionaltheory calculations with the standard 631G* and LanL2DZ basis sets are used for the evaluations of the relative production yields. The encapsulation Gibbs energy terms from the partition functions combined with the observed saturated metal pressures are employed. A model scheme is used dealing with the XS template gasphase presence. The computations predict that Sc2S@C82 should be produced in considerably larger amounts than Y2S@C82 which is in agreement with available observations. The yield order originates in the fact that both energetics and saturated metal pressure favor Sc over Y. © 2011 SpringerVerlag.
 Tung, W., Pavanello, M., & Adamowicz, L. (2011). Very accurate potential energy curve of the LiH molecule. Journal of Chemical Physics, 134(6).More infoPMID: 21322671;Abstract: We present very accurate calculations of the groundstate potential energy curve (PEC) of the LiH molecule performed with allelectron explicitly correlated Gaussian functions with shifted centers. The PEC is generated with the variational method involving simultaneous optimization of all Gaussians with an approach employing the analytical first derivatives of the energy with respect to the Gaussian nonlinear parameters (i.e., the exponents and the coordinates of the shifts). The LiH internuclear distance is varied between 1.8 and 40 bohrs. The absolute accuracy of the generated PEC is estimated as not exceeding 0.3 cm1. The adiabatic corrections for the four LiH isotopologues, i.e., 7LiH, 6LiH, 7LiD, and 6LiD, are also calculated and added to the LiH PEC. The aforementioned PECs are then used to calculate the vibrational energies for these systems. The maximum difference between the computed and the experimental vibrational transitions is smaller than 0.9 cm1. The contribution of the adiabatic correction to the dissociation energy of 7LiH molecule is 10.7 cm1. The magnitude of this correction shows its importance in calculating the LiH spectroscopic constants. As the estimated contribution of the nonadiabatic and relativistic effects to the ground state dissociation energy is around 0.3 cm1, their inclusion in the LiH PEC calculation seems to be the next most important contribution to evaluate in order to improve the accuracy achieved in this work. © 2011 American Institute of Physics.
 Adamowicz, L. (2010). Optimized virtual orbital space (OVOS) in coupledcluster calculations. Molecular Physics, 108(2123), 31053112.More infoAbstract: The optimized virtual orbital space (OVOS) method is described from the perspective of several developments that have occurred since the method was originally introduced 25 years ago during the author's postdoctoral stay in the research group of Rod Bartlett. We describe the OVOS approach in the context of the restricted HartreeFock (RHF), unrestricted HF (UHF), and multiconfiguration selfconsistent field (MCSCF) methods used to generate the reference zeroorder wave function for the OVOS calculations. © 2010 Taylor & Francis.
 Adamowicz, L., Tung, W., Pavanello, M., & Adamowicz, L. . (2010). Accurate onedimensional potential energy curve of the linear (H2)2 cluster. The Journal of chemical physics, 133(12).More infoWe present a sub0.3 K accuracy, groundstate onedimensional potential energy curve of the metastable linear configuration of the (H(2))(2) cluster calculated exclusively with explicitly correlated Gaussian functions with shifted centers. The H(2) internuclear distance is kept at the isolated H(2) vibrational groundstate average value of 1.448 736 bohr and the intermonomer separation is varied between 2 and 100 bohrs. The analytical gradient of the energy with respect to the nonlinear parameters of the Gaussians (i.e., the exponents and the coordinates of the shifts) has been employed in the variational optimization of the wave function. Procedures for enlarging the basis set and for adjusting the centers of the Gaussians to the varying intermonomer separation have been developed and used in the calculations.
 Bubin, S., Komasa, J., Stanke, M., & Adamowicz, L. (2010). Isotope shifts of the 1s22s2(1S0) → 1s22p2(1S0) transition in the doubly ionized carbon ion C^{2+}. Physical Review A  Atomic, Molecular, and Optical Physics, 81(5).More infoAbstract: Highly accurate quantum mechanical calculations are performed for the 1s22s2 (1S0) → 1s22p2 (1S0) transition energy in the isotopomers of C 2 ion to determine the isotope shifts. Explicitly correlated Gaussian functions and a variational approach that explicitly includes the nuclear motion are employed in the calculations. The leading relativistic and quantum electrodynamics corrections to the transition energy are also calculated using the perturbation theory with the nonrelativistic wave function as the zeroorder approximation. It is determined that the 12C2+ transitions energy, which is obtained from the calculations to be 182 519.031 cm1 (vs the experimental value of 182 519.88 cm1, an excellent subwavenumber agreement) upshifts by 1.755 cm1 for 13C 2+ and by additional 1.498 cm1 for 14C2+. Those shifts are sufficiently large to be measured experimentally. © 2010 The American Physical Society.
 Bubin, S., Komasa, J., Stanke, M., & Adamowicz, L. (2010). Isotope shifts of the three lowest S1 states of the B+ ion calculated with a finitenuclearmass approach and with relativistic and quantum electrodynamics corrections. Journal of Chemical Physics, 132(11).More infoPMID: 20331283;Abstract: We present very accurate quantum mechanical calculations of the three lowest S states [1 s2 2 s2 (1S0), 1 s2 2 p2 (1S 0), and 1 s2 2s3s (1S0)] of the two stable isotopes of the boron ion, 10B+ and 11B+. At the nonrelativistic level the calculations have been performed with the Hamiltonian that explicitly includes the finite mass of the nucleus as it was obtained by a rigorous separation of the centerofmass motion from the laboratory frame Hamiltonian. The spatial part of the nonrelativistic wave function for each state was expanded in terms of 10 000 allelectron explicitly correlated Gaussian functions. The nonlinear parameters of the Gaussians were variationally optimized using a procedure involving the analytical energy gradient determined with respect to the nonlinear parameters. The nonrelativistic wave functions of the three states were subsequently used to calculate the leading α2 relativistic corrections (α is the fine structure constant; α=1/c, where c is the speed of light) and the α3 quantum electrodynamics (QED) correction. We also estimated the α4 QED correction by calculating its dominant component. A comparison of the experimental transition frequencies with the frequencies obtained based on the energies calculated in this work shows an excellent agreement. The discrepancy is smaller than 0.4 cm1. © 2010 American Institute of Physics.
 Bubin, S., Stanke, M., & Adamowicz, L. (2010). Lower vibrational transitions of the ^{3}He^{4}He ^{+} ion calculated without the BornOppenheimer approximation and with leading relativistic corrections. Chemical Physics Letters, 500(46), 229231.More infoAbstract: Very accurate variational calculations of the five lowest vibrational states of the 3He4He+ ion are carried out within a framework that does not assume the BornOppenheimer (BO) approximation, i.e., treating the two nuclei and three electrons forming the system on an equal footing. The nonBO wave functions are expanded in terms of onecenter explicitly correlated Gaussian functions multiplied by even powers of the internuclear distance. The wave functions are used to calculate the leading relativistic corrections. The approach reproduces the experimental 3He4He+ fundamental transition within 0.055 cm1 and similar accuracy is expected for the higher yet unmeasured vibrational transitions determined in the present calculations. © 2010 Elsevier B.V. All rights reserved.
 Bubin, S., Stanke, M., Molski, M., & Adamowicz, L. (2010). Accurate nonBornOppenheimer calculations of the lowest vibrational energies of D_{2} and T_{2} with including relativistic corrections. Chemical Physics Letters, 494(13), 2125.More infoAbstract: In this work we report very accurate variational calculations of the two lowest vibrational states of the D2 and T2 molecules within the framework that does not assume the BornOppenheimer approximation. The nonrelativistic energies of the states obtained in the nonBO calculations are corrected for the relativistic effects of the order of α2 calculated as expectation values of the operators representing these effects. The ν=0→1 transition energy of D2 obtained in the calculations is compared with the transition frequency obtained from the experimental spectra. The comparison shows the need to include corrections higher than secondorder in α to further improve the agreement between the theory and the experiment. © 2010 Elsevier B.V. All rights reserved.
 Karachevtsev, V. A., Zarudnev, E. S., Stepanian, S. G., Glamazda, A. Y., Karachevtsev, M. V., & Adamowicz, L. (2010). Raman spectroscopy and theoretical characterization of nanohybrids of porphyrins with carbon nanotubes. Journal of Physical Chemistry C, 114(39), 1621516222.More infoAbstract: Singlewalled carbon nanotube (SWCNT) hybrids with meso5,10,15,20 tetrakis(Nmethyl4pyridyl)porphyrin (TMPyP4) and meso5,10,15,20 tetraphenylporphyrin (TPP) have been studied by the resonance Raman spectroscopy and by ab initio and molecular dynamic calculations. A comparison of the intensities of the bands assigned in the Raman spectrum to the radial breathing mode, as well as the relative band positions corresponding to the tangential modes of the hybrids with respect to the positions of these modes in the spectrum of the pristine SWCNT, indicates that the interaction of the nanotube with TMPyP4 is stronger than with TPP. A structure calculation (by the DFT/M052X method) of the TMPyP4 molecule adsorbed to a fragment of the zigzag (10,0) SWCNT surface shows that porphyrin adapts a saddled structure with the binding energy to the nanotube of 72.0 kcal/mol. The interaction energy in the complex of a TPP molecule with SWCNT, whose molecular structure is similar to TMPyP4, is only 19.3 kcal/mol and TPP does not adapt the saddled structure on the nanotube surface. The calculated interaction energies of the nanotube surface with the porphin core (14.3 kcal/mol), the charged fragment of TMPyP4 (methylpyridinium) (25.8 kcal/mol) and a neutral benzene molecule (4.6 kcal/mol), which mimic the side residues of TMPyP4 and TPP, respectively, suggest that the stronger interaction between SWCNT and the TMPyP4 molecule is due to the cationπ interaction. The SWCNT:TMPyP4 complex formation in the aqueous solution has been modeled by the molecular dynamics method. The modeling showed that the hybrid is also stable in the water solution. © 2010 American Chemical Society.
 Klimenko, T. A., Ivanov, V. V., Lyakh, D. I., & Adamowicz, L. (2010). A calculation of spectroscopic parameters for hydrogen fluoride with multireference statespecific coupledcluster method. Chemical Physics Letters, 493(13), 173178.More infoAbstract: The multireference statespecific coupledcluster method with a completeactivespace reference (CASCC) developed by our group has been used to calculate the potential energy curves, spectroscopic parameters, and vibrational levels for the ground (X1Σ+) and excited (B1Σ +) states of 19FH. The working algorithm for the CASCC method was derived with a computerbased automated approach that generates the CC energy and amplitude equations and the corresponding coupledcluster diagrams. The spinorbital sets for the CASCC calculations have been obtained using the completeactivespace selfconsistentfield method (CASSCF). As determined by comparing the theoretical results with experimental ones reconstructed using the RydbergKleinReese method, the CASCC energies are better than those obtained with the CCSD and EOMCCSD methods. © 2010 Elsevier B.V. All rights reserved.
 Pashynska, V., Boryak, O., Kosevich, M. V., Stepanian, S., & Adamowicz, L. (2010). Competition between counterions and active protein sites to bind bisquaternary ammonium groups. A combined mass spectrometry and quantum chemistry model study. European Physical Journal D, 58(3), 287296.More infoAbstract: A model study of the interaction between biologically active bisquaternary ammonium salts and their molecular targets in living systems is urgently needed to elucidate the molecular mechanisms involved in the interactions between these compounds. To address this need a combined experimentalcomputational study of the interaction of two tetramethylammonium cations (modeling two quaternary groups) with the chlorine anion and with the deprotonated 2,5dihydroxybenzoic acid (modeling a carboxylic group and an aromatic ring of side radicals of proteins) has been performed. Fast atom bombardment mass spectrometry method and DFT/B3LYP/631++G ** and MP2/631++G * * calculations have been employed in the study. Stable noncovalent complexes with different ratios of the tetramethylammonium cations and chlorine anions or deprotonated 2,5dihydroxybenzoic acid anions were registered in the mass spectra of tetramethylammonium chloride and 2,5dihydroxybenzoic acid mixture. This finding shows that the organic and inorganic anions compete to bind tetramethylammonium in the studied system. The theoretically determined stabilities of the noncovalent complexes were compared with the relative stabilities evaluated from the mass spectrometric measurements. The results of the study allow us to elucidate the competing interactions that exist between quaternary groups with inorganic counterions or with active groups of molecular protein targets.
 Pavanello, M., Adamowicz, L., Volobuyev, M., & Mennucci, B. (2010). Modeling hole transport in Wet and dry DNA. Journal of Physical Chemistry B, 114(13), 44164423.More infoPMID: 20235597;Abstract: We present a DFT/classical molecular dynamics model of DNA charge conductivity. The model involves a temperaturedriven, holehopping charge transfer and includes the timedependent nonequilibrium interaction of DNA with its molecular environment. We validate our method against a variety of hole transport experiments. The method predicts a significant holetransfer slowdown of ∼35% from dry to wet DNA with and without electric field bias. In addition, in agreement with experiments, it also predicts an insulating behavior of (GC)N oligomers for 40 < N < 1000, depending on the experimental setup. © 2010 American Chemical Society.
 Pavanello, M., Tung, W., & Adamowicz, L. (2010). Determination of deuteron quadrupole moment from calculations of the electric field gradient in D2 and HD. Physical Review A  Atomic, Molecular, and Optical Physics, 81(4).More infoAbstract: We have carried out an accurate determination of the quadrupole moment of the deuteron nucleus. The evaluation of the constant is achieved by combining high accuracy BornOppenheimer calculations of the electric field gradient at the nucleus in the H2 molecule with spectroscopic measurements of the quadrupolar splitting in D2 and HD. The derived value is Q=0.285783(30) fm2. © 2010 The American Physical Society.
 Sharkey, K. L., Bubin, S., & Adamowicz, L. (2010). Analytical energy gradient in variational calculations of the two lowest P3 states of the carbon atom with explicitly correlated Gaussian basis functions. Journal of Chemical Physics, 132(18).More infoAbstract: Variational calculations of ground and excited bound states on atomic and molecular systems performed with basis functions that explicitly depend on the interparticle distances can generate very accurate results provided that the basis function parameters are thoroughly optimized by the minimization of the energy. In this work we have derived the algorithm for the gradient of the energy determined with respect to the nonlinear exponential parameters of explicitly correlated Gaussian functions used in calculating n electron atomic systems with two p electrons and (n2) s electrons. The atomic Hamiltonian we used was obtained by rigorously separating out the kinetic energy of the center of mass motion from the laboratoryframe Hamiltonian and explicitly depends on the finite mass of the nucleus. The advantage of having the gradient available in the variational minimization of the energy is demonstrated in the calculations of the ground and the first excited P3 state of the carbon atom. For the former the lowest energy upper bound ever obtained is reported. © 2010 American Institute of Physics.
 Tung, W., Pavanello, M., & Adamowicz, L. (2010). Accurate onedimensional potential energy curve of the linear (H _{2})_{2} cluster. Journal of Chemical Physics, 133(12).More infoPMID: 20886923;Abstract: We present a sub0.3 K accuracy, groundstate onedimensional potential energy curve of the metastable linear configuration of the (H2)2 cluster calculated exclusively with explicitly correlated Gaussian functions with shifted centers. The H2 internuclear distance is kept at the isolated H2 vibrational groundstate average value of 1.448 736 bohr and the intermonomer separation is varied between 2 and 100 bohrs. The analytical gradient of the energy with respect to the nonlinear parameters of the Gaussians (i.e., the exponents and the coordinates of the shifts) has been employed in the variational optimization of the wave function. Procedures for enlarging the basis set and for adjusting the centers of the Gaussians to the varying intermonomer separation have been developed and used in the calculations. © 2010 American Institute of Physics.
 Adamowicz, L., Ivanov, V. V., Lyakh, D. I., & Adamowicz, L. . (2009). Multireference statespecific coupledcluster methods. Stateoftheart and perspectives. Physical chemistry chemical physics : PCCP, 11(14).More infoThis work reviews the statespecific multireference coupledcluster (CC) approaches which have been developed as approximate methods for performing highlevel quantum mechanical calculations on quasidegenerate ground and excited states of atomic and molecular systems. The term "quasidegenerate" refers to a state that cannot be described even in the first approximation by a singledeterminant wavefunction (a Slater determinant), but requires two or more determinants for this purpose. The main challenge with applying the coupledcluster theory to such states is in describing the electron correlation effects in the wavefunctions representing these states in a manner that is sizeextensive, yet accurate and simple enough so the method can be routinely applied to small and mediumsize molecular systems. We are describing how this can be accomplished within a theory that focuses on only one state of the system in a single CC calculation (the statespecific theory).
 Adamowicz, L., Pavanello, M., & Adamowicz, L. . (2009). Highaccuracy calculations of the ground, 1 1A1', and the 2 1A1', 2 3A1', and 1 1E' excited states of H3+. The Journal of chemical physics, 130(3).More infoAccurate variational BornOppenheimer calculations of the 1 (1)A(1) ('), 2 (1)A(1) ('), 2 (3)A(1) ('), and 1 (1)E(') states of the H(3) (+) ion at the groundstate equilibrium geometry are reported. The wave functions of the states are expanded in terms of explicitly correlated spherical Gaussian functions with shifted centers. In the variational optimization the analytical gradient of the energy with respect to the nonlinear exponential parameters of the Gaussians has been employed. The energies obtained in the calculations are the best variational estimates ever calculated for the four states. Oneelectron densities for the states, as well as a D(3h)restricted potential energy surface of the ground state calculated around the equilibrium geometry, are also presented and discussed.
 Adamowicz, L., Pavanello, M., Tung, W., & Adamowicz, L. . (2009). How to calculate H3 better. The Journal of chemical physics, 131(18).More infoEfficient optimization of the basis set is key to achieving a very high accuracy in variational calculations of molecular systems employing basis functions that are explicitly dependent on the interelectron distances. In this work we present a method for a systematic enlargement of basis sets of explicitly correlated functions based on the iterativecomplementinteraction approach developed by Nakatsuji [Phys. Rev. Lett. 93, 030403 (2004)]. We illustrate the performance of the method in the variational calculations of H(3) where we use explicitly correlated Gaussian functions with shifted centers. The total variational energy (1.674 547 421 Hartree) and the binding energy (15.74 cm(1)) obtained in the calculation with 1000 Gaussians are the most accurate results to date.
 Adamowicz, L., Pavanello, M., Tung, W., Leonarski, F., & Adamowicz, L. . (2009). New more accurate calculations of the ground state potential energy surface of H(3) (+). The Journal of chemical physics, 130(7).More infoExplicitly correlated Gaussian functions with floating centers have been employed to recalculate the ground state potential energy surface (PES) of the H(3) (+) ion with much higher accuracy than it was done before. The nonlinear parameters of the Gaussians (i.e., the exponents and the centers) have been variationally optimized with a procedure employing the analytical gradient of the energy with respect to these parameters. The basis sets for calculating new PES points were guessed from the points already calculated. This allowed us to considerably speed up the calculations and achieve very high accuracy of the results.
 Ard, S., Garrett, W. R., Compton, R. N., Adamowicz, L., & Stepanian, S. G. (2009). Rotational states of dipolebound anions of hydrogen cyanide. Chemical Physics Letters, 473(46), 223226.More infoAbstract: Rotational states of dipolebound anions of HCN are studied both experimentally and theoretically. The ground state experimental electron affinity (ea), 1.56 meV, is in good agreement with present and one earlier theoretical value. The predicted binding energy for the J = 1 state, 1.2 meV, is observed, whereas J = 2 is too fragile for observation and J = 3 is unbound. © 2009 Elsevier B.V. All rights reserved.
 Bubin, S., & Adamowicz, L. (2009). Calculations of lowlying P1 states of the beryllium atom. Physical Review A  Atomic, Molecular, and Optical Physics, 79(2).More infoAbstract: Highaccuracy nonrelativistic variational calculations employing explicitly correlated Gaussian basis functions have been performed to determine the energies and the expectation values of some operators for the lowest four P11 states of the beryllium atom. The states correspond to the electron configurations 1 s2 2 s1 n p1, where n=2, 3, 4, and 5. The calculations were performed for both finite and infinite mass of the Be nucleus. The basis set for each state was grown to the level of 5000 Gaussians. With that many functions we achieved a tight energy convergence. The reported values, to the best of our knowledge, are the most accurate ever obtained for the four states. © 2009 The American Physical Society.
 Bubin, S., Komasa, J., Stanke, M., & Adamowicz, L. (2009). Isotope shift in the electron affinity of lithium. Journal of Chemical Physics, 131(23).More infoPMID: 20025319;Abstract: Very accurate electron affinity (EA) calculations of 6Li and 7Li (and ∞Li) have been performed using explicitly correlated Gaussian functions and a variational approach that explicitly includes the nuclear motion in the calculations (i.e., the approach that does not assume the BornOppenheimer approximation). The leading relativistic and quantum electrodynamics corrections to the electron affinities were also calculated. The results are the most accurate theoretical values obtained for the studied systems to date. Our best estimates of the 7Li and 6Li EAs are 4984.9842(30) and 4984.9015 (30) cm1, respectively, and of the 7Li/6Li EA isotope shift is 0.0827 cm1. © 2009 American Institute of Physics.
 Bubin, S., Leonarski, F., Stanke, M., & Adamowicz, L. (2009). Charge asymmetry in pure vibrational states of the HD molecule. Journal of Chemical Physics, 130(12).More infoPMID: 19334821;Abstract: Very accurate variational calculations of all rotationless states (also called pure vibrational states) of the HD molecule have been performed within the framework that does not assume the BornOppenheimer (BO) approximation. The nonBO wave functions of the states describing the internal motion of the proton, the deuteron, and the two electrons were expanded in terms of onecenter explicitly correlated Gaussian functions multiplied by even powers of the internuclear distance. Up to 6000 functions were used for each state. Both linear and nonlinear parameters of the wave functions of all 18 states were optimized with a procedure that employs the analytical gradient of the energy with respect to the nonlinear parameters of the Gaussians. These wave functions were used to calculate expectation values of the interparticle distances and some other related quantities. The results allow elucidation of the charge asymmetry in HD as a function of the vibrational excitation. © 2009 American Institute of Physics.
 Bubin, S., Leonarski, F., Stanke, M., & Adamowicz, L. (2009). Nonadiabatic corrections to the energies of the pure vibrational states of H_{2}. Chemical Physics Letters, 477(13), 1216.More infoAbstract: Nonrelativistic energies of all fifteen pure vibrational states of the H2 molecule have been recalculated with much higher accuracy than before. In the calculations we employed explicitly correlated Gaussian functions and an approach where the BornOppenheimer (BO) approximation is not assumed. The wave function of each state was expanded in terms of 10 000 Gaussians whose nonlinear parameters were optimized using a procedure involving the analytical energy gradient. The obtained nonBO energies combined with the recent BO adiabatic energies of Pachucki and Komasa [K. Pachucki, J. Komasa, J. Chem. Phys. 129 (2008) 034102] allowed us to determine new improved values of the nonadiabatic corrections for the considered states. © 2009 Elsevier B.V. All rights reserved.
 Bubin, S., Stanke, M., & Adamowicz, L. (2009). NonBornOppenheimer calculations of the BH molecule. Journal of Chemical Physics, 131(4).More infoPMID: 19655858;Abstract: Variational calculations employing explicitly correlated Gaussian basis functions have been performed for the ground state of the boron monohydride molecule (BH) and for the boron atom (B). Up to 2000 Gaussians were used for each system. The calculations did not assume the BornOppenheimer (BO) approximation. In the optimization of the wave function, we employed the analytical energy gradient with respect to the Gaussian exponential parameters. In addition to the total nonrelativistic energies, we computed scalar relativistic corrections (massvelocity and Darwin). With those added to the total energies, we estimated the dissociation energy of BH. The nonBO wave functions were also used to compute some expectation values involving operators dependent on the interparticle distances. © 2009 American Institute of Physics.
 Ivanov, V. V., Lyakh, D. I., & Adamowicz, L. (2009). Multireference statespecific coupledcluster methods. Stateoftheart and perspectives. Physical Chemistry Chemical Physics, 11(14), 23552370.More infoPMID: 19325966;Abstract: This work reviews the statespecific multireference coupledcluster (CC) approaches which have been developed as approximate methods for performing highlevel quantum mechanical calculations on quasidegenerate ground and excited states of atomic and molecular systems. The term "quasidegenerate" refers to a state that cannot be described even in the first approximation by a singledeterminant wavefunction (a Slater determinant), but requires two or more determinants for this purpose. The main challenge with applying the coupledcluster theory to such states is in describing the electron correlation effects in the wavefunctions representing these states in a manner that is sizeextensive, yet accurate and simple enough so the method can be routinely applied to small and mediumsize molecular systems. We are describing how this can be accomplished within a theory that focuses on only one state of the system in a single CC calculation (the statespecific theory). © the Owner Societies 2009.
 Klimenko, T. A., Ivanov, V. V., & Adamowicz, L. (2009). Dipole polarizabilities and hyperpolarizabilities of the small conjugated systems in the πelectron coupled cluster theory. Molecular Physics, 107(17), 17291737.More infoAbstract: In this work, πelectron dipole polarizabilities and hyperpolarizabilities of some organic systems with double and triple bonds (polyenes, polyynes, and condensed hydrocarbons) have been calculated using the coupledcluster (CC) πelectron method. It is shown that the CC approach gives the polarizabilities and hyperpolarizabilities which are considerably closer to the full configuration interaction (FCI) results than the results of the HartreeFock method and the second order perturbation theory. A correct description of (hyper)polarizabilities of polyenes can be achieved at the CC level of theory with the inclusion of only single and double electron excitations from the HartreeFock determinant (the CCSD approach). However, for systems with triple bonds (acetylenes), higher level excitations need to be explicitly included in the calculations to correctly describe their (hyper)polarizabilities. © 2009 Taylor & Francis.
 Pavanello, M., & Adamowicz, L. (2009). Highaccuracy calculations of the ground, 1 ^{1}A _{1}′, and the 2 ^{1}A_{1}′,2 ^{3}A_{1}′, and 1 ^{1}E′, excited states of H_{3}^{+}. Journal of Chemical Physics, 130(3).More infoPMID: 19173507;Abstract: Accurate variational BornOppenheimer calculations of the 1 1A1′, 2 1A1′,2 3A1′, and 1 1E′ states of the H3+ ion at the groundstate equilibrium geometry are reported. The wave functions of the states are expanded in terms of explicitly correlated spherical Gaussian functions with shifted centers. In the variational optimization the analytical gradient of the energy with respect to the nonlinear exponential parameters of the Gaussians has been employed. The energies obtained in the calculations are the best variational estimates ever calculated for the four states. Oneelectron densities for the states, as well as a D3h restricted potential energy surface of the ground state calculated around the equilibrium geometry, are also presented and discussed. © 2009 American Institute of Physics.
 Pavanello, M., Tung, W., & Adamowicz, L. (2009). How to calculate H_{3} better. Journal of Chemical Physics, 131(18).More infoPMID: 19916597;Abstract: Efficient optimization of the basis set is key to achieving a very high accuracy in variational calculations of molecular systems employing basis functions that are explicitly dependent on the interelectron distances. In this work we present a method for a systematic enlargement of basis sets of explicitly correlated functions based on the iterativecomplementinteraction approach developed by Nakatsuji [Phys. Rev. Lett. 93, 030403 (2004)]. We illustrate the performance of the method in the variational calculations of H3 where we use explicitly correlated Gaussian functions with shifted centers. The total variational energy (1.674 547 421 Hartree) and the binding energy (15.74 cm1) obtained in the calculation with 1000 Gaussians are the most accurate results to date. © 2009 American Institute of Physics.
 Pavanello, M., Tung, W., Leonarski, F., & Adamowicz, L. (2009). New more accurate calculations of the ground state potential energy surface of H_{3}^{+}. Journal of Chemical Physics, 130(7).More infoPMID: 19239282;Abstract: Explicitly correlated Gaussian functions with floating centers have been employed to recalculate the ground state potential energy surface (PES) of the H3+ ion with much higher accuracy than it was done before. The nonlinear parameters of the Gaussians (i.e., the exponents and the centers) have been variationally optimized with a procedure employing the analytical gradient of the energy with respect to these parameters. The basis sets for calculating new PES points were guessed from the points already calculated. This allowed us to considerably speed up the calculations and achieve very high accuracy of the results. © 2009 American Institute of Physics.
 Sharkey, K. L., Pavanello, M., Bubin, S., & Adamowicz, L. (2009). Algorithm for quantummechanical finitenuclearmass variational calculations of atoms with two p electrons using allelectron explicitly correlated Gaussian basis functions. Physical Review A  Atomic, Molecular, and Optical Physics, 80(6).More infoAbstract: A new algorithm for calculating the Hamiltonian matrix elements with allelectron explicitly correlated Gaussian functions for quantummechanical calculations of atoms with two p electrons or a single d electron have been derived and implemented. The Hamiltonian used in the approach was obtained by rigorously separating the centerofmass motion and it explicitly depends on the finite mass of the nucleus. The approach was employed to perform test calculations on the isotopes of the carbon atom in their ground electronic states and to determine the finitenuclearmass corrections for these states. © 2009 The American Physical Society.
 Stanke, M., Bubin, S., & Adamowicz, L. (2009). Fundamental vibrational transitions of the H3 e H4 e+ and L7 iH+ ions calculated without assuming the BornOppenheimer approximation and with including leading relativistic corrections. Physical Review A  Atomic, Molecular, and Optical Physics, 79(6).More infoAbstract: Very accurate variational calculations of the fundamental pure vibrational transitions of the H3 e H4 e+ and L7 iH+ ions are performed within the framework that does not assume the BornOppenheimer (BO) approximation. The nonBO wave functions expanded in terms of onecenter explicitly correlated Gaussian functions multiplied by even powers of the internuclear distance are used to calculate the leading relativistic corrections. Up to 10000 Gaussian functions are used for each state. It is shown that the experimental H3 e H4 e+ fundamental transitions is reproduced within 0.06 cm1 by the calculations. A similar precision is expected for the calculated, but still unmeasured, fundamental transition of L7 iH+. Thus, threeelectron diatomic systems are calculated with a similar accuracy as twoelectron systems. © 2009 The American Physical Society.
 Stanke, M., Bubin, S., Molski, M., & Adamowicz, L. (2009). NonBornOppenheimer calculations of the lowest vibrational energy of HD including relativistic corrections. Physical Review A  Atomic, Molecular, and Optical Physics, 79(3).More infoAbstract: In this work we report variational calculations of the two lowest vibrational states of the HD molecule within the framework that does not assume the BornOppenheimer (BO) approximation. The nonrelativistic energies of the states were corrected for the relativistic effects of the order of α2 (where α= 1 c), calculated as expectation values of the operators representing these effects with the nonrelativistic nonBO wave functions. The nonBO wave functions were expanded in terms of the onecenter explicitly correlated Gaussian functions multiplied by even powers of the internuclear distance. The v=0→1 transition energy obtained in the calculations is compared with the previous calculations, as well as with the transition frequency obtained from the experimental spectra. The comparison shows the need to include corrections higher than second order in α to further improve the agreement between the theory and the experiment. © 2009 The American Physical Society.
 Stanke, M., Komasa, J., Bubin, S., & Adamowicz, L. (2009). Five lowest S1 states of the Be atom calculated with a finitenuclearmass approach and with relativistic and QED corrections. Physical Review A  Atomic, Molecular, and Optical Physics, 80(2).More infoAbstract: We have performed very accurate quantum mechanical calculations of the five lowest S states of the beryllium atom. In the nonrelativistic part of the calculations we used the variational method and we explicitly included the nuclear motion in the Schrödinger equation. The nonrelativistic wave functions of the five states were expanded in terms of explicitly correlated Gaussian functions. These wave functions were used to calculate the leading α2 relativistic correction (α is the fine structure constant) and the α3 quantum electrodynamics (QED) correction. We also estimated the α4 QED correction by calculating its dominant component. A comparison of the experimental transition frequencies with the frequencies obtained based on the energies calculated in this work shows an excellent agreement. © 2009 The American Physical Society.
 Stepanian, S. G., Karachevtsev, M. V., Glamazda, A. Y., Karachevtsev, V. A., & Adamowicz, L. (2009). Raman spectroscopy study and firstprinciples calculations of the interaction between nucleic acid bases and carbon nanotubes. Journal of Physical Chemistry A, 113(15), 36213629.More infoPMID: 19320448;Abstract: In this work, we have used Raman spectroscopy and quantum chemical methods (MP2 and DFT) to study the interactions between nucleic acid bases (NABs) and singlewalled carbon nanotubes (SWCNT). We found that the appearance of the interaction between the nanotubes and the NABs is accompanied by a spectral shift of the highfrequency component of the SWCNT G band in the Raman spectrum to a lower frequency region. The value of this shift varies from 0.7 to 1.3 cm 1 for the metallic nanotubes and from 2.1 to 3.2 cm 1 for the semiconducting nanotubes. Calculations of the interaction energies between the NABs and a fragment of the zigzag(10,0) carbon nanotube performed at the MP2/631++G(d,p)[NABs atoms] 163 IG(d) [nanotube atoms] level of theory while accounting for the basis set superposition error during geometry optimization allowed us to order the NABs according to the increasing interaction energy value. The order is: guanine (67.1 kJ mol 1) > adenine (59.0 kJ mol 1) > cytosine (50.3 kJ mol 1) % thymine (50.2 kJ mol 1) > uracil (44.2 kJ mol 1). The MP2 equilibrium structures and the interaction energies were used as reference points in the evaluation of the ability of various functionals in the DFT method to predict those structures and energies. We showed that the M05, MPWBlK, and MPW1B95 density functionals are capable of correctly predicting the SWCNTNAB geometries but not the interaction energies, while the M052X functional is capable of correctly predicting both the geometries and the interaction energies. © 2009 American Chemical Society.
 Trzaskowski, B., & Adamowicz, L. (2009). Chloromethane and dichloromethane decompositions inside nanotubes as models of reactions in confined space. Theoretical Chemistry Accounts, 124(12), 95103.More infoAbstract: A combination of ab initio MP2 and molecular mechanics UFF calculations have been employed to study chloromethane and dichloromethane decomposition reaction inside carbon nanotubes (CNTs). The results suggest that the impact of the nanotubes on the mechanism of the reaction depends on the diameter of the nanotube. Nanotubes with a large diameter affect the reaction in a negligible way. On the other hand, most of the reactions taking place inside small nanotubes are considerably altered. The presence of the CNT may affect the geometries of the reactants, the reaction energy barriers, as well as the energetic outcome of the reactions. All the reactions have been described by means of energetic, thermodynamic, and vibrational analyses, which allowed us to form general conclusions concerning the reaction taking place in a confined space. © SpringerVerlag 2009.
 Adamowicz, L., Bubin, S., & Adamowicz, L. . (2008). Energy and energy gradient matrix elements with Nparticle explicitly correlated complex Gaussian basis functions with L=1. The Journal of chemical physics, 128(11).More infoIn this work we consider explicitly correlated complex Gaussian basis functions for expanding the wave function of an Nparticle system with the L=1 total orbital angular momentum. We derive analytical expressions for various matrix elements with these basis functions including the overlap, kinetic energy, and potential energy (Coulomb interaction) matrix elements, as well as matrix elements of other quantities. The derivatives of the overlap, kinetic, and potential energy integrals with respect to the Gaussian exponential parameters are also derived and used to calculate the energy gradient. All the derivations are performed using the formalism of the matrix differential calculus that facilitates a way of expressing the integrals in an elegant matrix form, which is convenient for the theoretical analysis and the computer implementation. The new method is tested in calculations of two systems: the lowest P state of the beryllium atom and the bound P state of the positronium molecule (with the negative parity). Both calculations yielded new, lowesttodate, variational upper bounds, while the number of basis functions used was significantly smaller than in previous studies. It was possible to accomplish this due to the use of the analytic energy gradient in the minimization of the variational energy.
 Adamowicz, L., Lyakh, D. I., Ivanov, V. V., & Adamowicz, L. . (2008). A generalization of the statespecific completeactivespace coupledcluster method for calculating electronic excited states. The Journal of chemical physics, 128(7).More infoThe completeactivespace coupledcluster approach with single and double excitations (CASCCSD) based on the ansatz of Oliphant and Adamowicz [J. Chem. Phys. 94, 1229 (1991); 96, 3739 (1992)] is used to derive an approach termed XCASCCSD for calculating potential energy surfaces of ground and excited electronic states with different multiplicities and symmetries. The XCASCCSD approach explicitly includes a procedure for spin and spatial orbitalmomentum symmetry adaptation of the wave function that has allowed us to consider states with degenerate formal references. The XCASCCSD method is applied to calculate potential energy surfaces of the ground and some lowest singlet and triplet excited states of the FH and C(2) molecules. Some states of C(2) are known to have a very strong "multireference" character making their description difficult with singlereference methods. The problem of the change of the formal reference determinant along the potential energy surface is discussed. Also, vertical excitation energies of formaldehyde calculated with the XCASCCSD approach are presented.
 Adamowicz, L., Stanke, M., Kedziera, D., Bubin, S., Molski, M., & Adamowicz, L. . (2008). Orbitorbit relativistic corrections to the pure vibrational nonBornOppenheimer energies of H(2). The Journal of chemical physics, 128(11).More infoWe report the derivation of the orbitorbit relativistic correction for calculating pure vibrational states of diatomic molecular systems with sigma electrons within the framework that does not assume the BornOppenheimer (BO) approximation. The correction is calculated as the expectation value of the orbitorbit interaction operator with the nonBO wave function expressed in terms of explicitly correlated Gaussian functions multiplied by even powers of the internuclear distance. With that we can now calculate the complete relativistic correction of the order of alpha(2) (where alpha=1/c). The new algorithm is applied to determine the full set of the rotationless vibrational levels and the corresponding transition frequencies of the H(2) molecule. The results are compared with the previous calculations, as well as with the frequencies obtained from the experimental spectra. The comparison shows the need to include corrections higher than second order in alpha to further improve the agreement between the theory and the experiment.
 Adamowicz, L., Trzaskowski, B., Leonarski, F., Leś, A., & Adamowicz, L. . (2008). Altering the orientation of proteins on selfassembled monolayers: a computational study. Biomacromolecules, 9(11).More infoA combined computational dockingmolecular dynamics study has been performed on a system consisting of cytochrome c protein and alkanethiol selfassembled monolayers of various geometries. The results suggest that the orientation of the protein on the surface may be controlled or altered by means of designing specific structural motifs on the surface. The proposed computational approach may be used as a fast and reliable tool to complement other theoretical and experimental techniques of exploring other proteinsurface interfaces.
 Bubin, S., & Adamowicz, L. (2008). Energy and energy gradient matrix elements with N particle explicitly correlated complex Gaussian basis functions with L=1. Journal of Chemical Physics, 128(11).More infoPMID: 18361554;Abstract: In this work we consider explicitly correlated complex Gaussian basis functions for expanding the wave function of an N particle system with the L=1 total orbital angular momentum. We derive analytical expressions for various matrix elements with these basis functions including the overlap, kinetic energy, and potential energy (Coulomb interaction) matrix elements, as well as matrix elements of other quantities. The derivatives of the overlap, kinetic, and potential energy integrals with respect to the Gaussian exponential parameters are also derived and used to calculate the energy gradient. All the derivations are performed using the formalism of the matrix differential calculus that facilitates a way of expressing the integrals in an elegant matrix form, which is convenient for the theoretical analysis and the computer implementation. The new method is tested in calculations of two systems: the lowest P state of the beryllium atom and the bound P state of the positronium molecule (with the negative parity). Both calculations yielded new, lowesttodate, variational upper bounds, while the number of basis functions used was significantly smaller than in previous studies. It was possible to accomplish this due to the use of the analytic energy gradient in the minimization of the variational energy. © 2008 American Institute of Physics.
 Jalbout, A. F., ContrerasTorres, F. F., & Adamowicz, L. (2008). The hydroxyacetone (CH _{3}COCH _{2}(OH)) torsional potential and isomerization: A theoretical study. International Journal of Quantum Chemistry, 108(2), 279288.More infoAbstract: In this work we explore the torsional potentials for hydroxyacetone using second order NtellerPlesset (MP2) and coupledcluster (CCSD(T)) ab initio theoretical methods. We have calculated these potential energy surfaces as a function of single OH rotation, and simultaneous OH and methyl rotation. The results indicate that the low energy isomers coupled to the reduced barrier to methyl rotation can lead to several rotamers for this species to form. The values are compared to experimental quantities and implications for astrobiological research are also addressed. © 2007 Wiley Periodicals, Inc.
 Jalbout, A. F., Trzaskowski, B., Xia, Y., Li, Y., Hu, X., Li, H., ElNahas, A., & Adamowicz, L. (2008). Erratum to "Structures, stabilities and tautomerizations of uracil and diphosphouracil tautomers" [Chem. Phys. 332 (2007) 152161] (DOI:10.1016/j.chemphys.2006.10.026). Chemical Physics, 348(13), 254.
 Karachevtsev, M. V., Lytvyn, O. S., Stepanian, S. G., Leontiev, V. S., Adamowicz, L., & Karachevtsev, V. A. (2008). SWNTDNA and SWNTpolyC hybrids: AFM study and computer modeling. Journal of Nanoscience and Nanotechnology, 8(3), 14731480.More infoPMID: 18468177;Abstract: Hybrids of carbon singlewalled nanotubes (SWNT) with fragmented single or doublestranded DNA (fss or fdsDNA) or polyC were studied by Atom Force Microscopy (AFM) and computer modeling. It was found that fragments of the polymer wrap in several layers around the nanotube, forming a strandlike spindle. In contrast to the fssDNA, the fdsDNA also forms compact structures near the tube surface due to the formation of selfassembly structures consisting of a few DNA fragments. The hybrids of SWNT with wrapped single, double or triple strands of the biopolymer were simulated, and it was shown that such structures are stable. To explain the reason of multilayer polymeric coating of the nanotube surface, the energy of the intermolecular interactions between different components of polyC was calculated at the MP2/631 ++G** level as well as the interaction energy in the SWNTcytosine complex. Copyright © 2008 American Scientific Publishers All rights reserved.
 Karachevtsev, V. A., Zarudnev, E. S., Glamazda, A., Plokhotnichenko, A. M., Zozulya, V. N., Stepanian, S. G., & Adamowicz, L. (2008). Preresonance Raman and IR absorption spectroscopy of imidazophenazine and its derivatives: Experimental and ab initio study. Vibrational Spectroscopy, 47(2), 7181.More infoAbstract: The preresonance Raman and IR absorption spectroscopy techniques have been applied to study the influence of substitutes in the imidazole ring of imidazo[4,5d]phenazine (F1) on its vibrational structure. The following F1 derivatives in the KBr pellets have been investigated: 2methylimidazo[4,5d]phenazine (F2), 2trifluoridemethylimidazo[4,5d]phenazine (F3), and 1,2,3triazole[4,5d]phenazine (F4). The experimental Raman and IR data were analyzed using the harmonic frequencies, the IR intensities, and the preresonance Raman activities calculated at the DFT(B3LYP)/631++G** level of theory. We found a good agreement between the observed and the calculated spectra for all the normal modes except the ones attributed to the vibrations of the NH group. These latter modes are the most affected by the strong intermolecular interactions that occur in the solid samples. The bending ring vibrations dominate in the spectra of all compounds studied. The vibrational structure of the F1 derivatives is similar to that of the parent compound, though for each compound, characteristic bands also appear in the spectrum. Among the F1 derivatives, the vibrational spectrum of the F4 compound where the C atom of the imidazole ring is replaced by the N atom, changes the most from the spectrum of the parent compound. The nature of the six lowest electronic transitions to the S1S6 states in the F1F4 compounds and the forms of the π, π* and n orbitals involved in the transitions were also determined. This revealed that the first singlet excited state in all studied compounds corresponds to a π → π* transition. © 2008 Elsevier B.V. All rights reserved.
 Lyakh, D. I., Ivanov, V. V., & Adamowicz, L. (2008). A generalization of the statespecific completeactivespace coupledcluster method for calculating electronic excited states. Journal of Chemical Physics, 128(7).More infoPMID: 18298134;Abstract: The completeactivespace coupledcluster approach with single and double excitations (CASCCSD) based on the ansatz of Oliphant and Adamowicz [J. Chem. Phys. 94, 1229 (1991); 96, 3739 (1992)] is used to derive an approach termed XCASCCSD for calculating potential energy surfaces of ground and excited electronic states with different multiplicities and symmetries. The XCASCCSD approach explicitly includes a procedure for spin and spatial orbitalmomentum symmetry adaptation of the wave function that has allowed us to consider states with degenerate formal references. The XCASCCSD method is applied to calculate potential energy surfaces of the ground and some lowest singlet and triplet excited states of the FH and C2 molecules. Some states of C2 are known to have a very strong "multireference" character making their description difficult with singlereference methods. The problem of the change of the formal reference determinant along the potential energy surface is discussed. Also, vertical excitation energies of formaldehyde calculated with the XCASCCSD approach are presented. © 2008 American Institute of Physics.
 Pavanello, M., Cafiero, M., Bubin, S., & Adamowicz, L. (2008). Accurate BornOppenheimer calculations of the lowlying c ^{3}∑_{g}^{+} and a^{3}∑ _{u}^{+} excited states of helium dimer. International Journal of Quantum Chemistry, 108(12 SPEC. ISS.), 22912298.More infoAbstract: Accurate variational BornOppenheimer calculations of the two lowest excited triplet states of the helium dimer at their respective equilibrium geometries are reported. The wave functions of the states are expanded in terms of explicitly correlated Gaussian functions with shifted centers. The obtained energies are the best variational estimates ever calculated for these states. Oneelectron densities are also presented and discussed. The results are compared with the experimental values and previous calculations. © 2008 Wiley Periodicals, Inc.
 Slanina, Z., Uhlík, F., Lee, S., Adamowicz, L., & Nagase, S. (2008). Computational screening of metallofullerenes for nanoscience: Sr@C74. Molecular Simulation, 34(1), 1721.More infoAbstract: Densityfunctional theory calculations are presented for Sr@C74 where C74 is either the isolated pentagon rule (IPR) cage or two cages with a pentagonpentagon junction. Their relative thermodynamic production yields are evaluated using the calculated terms and it is shown that the IPRbased endohedral prevails at relevant temperatures. The encapsulation Gibbs energy is also combined with the saturated metal pressure as the combined term allows for evaluation of the relative production yields in a metallofullerene series.
 Slanina, Z., Uhlík, F., Lee, S., Adamowicz, L., & Nagase, S. (2008). Computations on three isomers of La@C_{74}. International Journal of Quantum Chemistry, 108(14), 26362640.More infoAbstract: Densityfunctional theory calculations are presented for La@C74, where C74 is either the IPR (isolated pentagon rule) cage or two cages with a pentagonpentagon junction. Their relative thermodynamic production yields are evaluated using the calculated terms, and it is shown that the IPRbased endohedral prevails at relevant temperatures in agreement with the observation. © 2008 Wiley Periodicals, Inc.
 Slanina, Z., Uhlík, F., Lee, S., Adamowicz, L., & Nagase, S. (2008). Li _{x}@C _{60}: Calculations of the encapsulation energetics and thermodynamics. International Journal of Molecular Sciences, 9(9), 18411850.More infoPMID: 19325833;PMCID: PMC2635758;Abstract: Li@C 60 and Li@C 70 can be prepared and thus, their calculations at higher levels of theory are also of interest. In the report, the computations are carried out on Li@C 60, Li 2 @C 60 and Li 3@C 60 with the B3LYP densityfunctional theory treatment in the standard 321G and 631G* basis sets. The computed energetics suggests that Li x@C 60 species may be produced for a few small x values if the Li pressure is enhanced sufficiently. In order to check the suggestion, a deeper computational evaluation of the encapsulation thermodynamics is carried out. © 2008 by the authors.
 Slanina, Z., Uhlík, F., Lee, S., Adamowicz, L., & Nagase, S. (2008). MPWB1K calculations of stepwise encapsulations: Li_{x}@C_{60}. Chemical Physics Letters, 463(13), 121123.More infoAbstract: Computations are carried out on Li@C60, Li2@C60 and Li3@C60 with the newly introduced MPWB1K density functional. The computed equilibrium thermodynamics shows that Li2@C60 is at least by two orders of magnitude less populated than Li@C60 when the saturated Li vapor is considered while for Li3@C60 it is at least four orders of magnitude compared to Li@C60. The evaluations correspond to the available experimental findings. © 2008 Elsevier B.V. All rights reserved.
 Slanina, Z., Uhlík, F., Sheu, J., Lee, S., Adamowicz, L., & Nagase, S. (2008). Stabilities of fullerenes: Illustration on C_{80}. Match, 59(1), 225238.More infoAbstract: Theoretical studies of fullerenes, metallofullerenes, and their derivatives have employed a wide spectrum of techniques, in particular graphtheoretical (like the Wiener index), quantumchemical, and statisticalmechanical treatments, in order to evaluate and rationalize stability, reactivity, and properties of the unique cage species. The theoretical stability measures of various types are analyzed, compared and illustrated on C80 cages satisfying the isolatedpentagon rule.
 Stanke, M., Komasa, J., Kdziera, D., Bubin, S., & Adamowicz, L. (2008). Accuracy limits on the description of the lowest S excitation in the Li atom using explicitly correlated Gaussian basis functions. Physical Review A  Atomic, Molecular, and Optical Physics, 78(5).More infoAbstract: We have performed very accurate quantummechanical calculations for the two lowest S states of the lithium atom in order to determine the transition energy. In the nonrelativistic part of the calculations performed using the variational method, we explicitly included the nuclear motion. The nonrelativistic wave function was expanded in terms of explicitly correlated Gaussian functions. Next, this wave function was used to calculate the leading α2 relativistic correction (α is the finestructure constant) and the α3 QED correction. We also estimated the α4 QED correction by calculating its dominating component. The results obtained with Gaussians are compared with the most accurate results obtained recently with the Hylleraastype basis functions. © 2008 The American Physical Society.
 Stanke, M., Komasa, J., Kȩdziera, D., Bubin, S., & Adamowicz, L. (2008). Three lowest S states of B9 e+ calculated with including nuclear motion and relativistic and QED corrections. Physical Review A  Atomic, Molecular, and Optical Physics, 77(6).More infoAbstract: We have performed highaccuracy quantum mechanical calculations for the three lowest S states of the beryllium ion (B9 e+). The nonrelativistic part of the calculations was done with the variational approach and explicitly included the nuclear motion (i.e., the finitenuclearmass approach). The nonrelativistic wave functions were expanded in terms of explicitly correlated Gaussian functions. These nonrelativistic functions were subsequently used to calculate the leading α2 relativistic corrections (α=1/c) and the α3 and α4 QED (quantum electrodynamics) corrections. In the α4 QED correction we only accounted for its dominant component typically contributing about 80% of the correction. With those the present results are the most accurate ever obtained for B9 e+. They also agree with the experimentally measured transitions within less than 0.1 cm1. © 2008 The American Physical Society.
 Stanke, M., Kȩdziera, D., Bubin, S., & Adamowicz, L. (2008). Complete I 2 relativistic corrections to the pure vibrational nonBornOppenheimer energies of HeH+. Physical Review A  Atomic, Molecular, and Optical Physics, 77(2).More infoAbstract: We report the implementation of the complete set of the lowestorder relativistic corrections of the order of Î±2 (where Î± is the fine structure constant) for calculating vibrational states of diatomic molecular systems within the framework that does not assume the BornOppenheimer approximation. To test the accuracy of the approach we have performed calculations for all rotationless vibrational states (also called pure vibrational states or S states) of the HeH+ ion in the ground electronic state. For the lowest transitions, where very precise experimental results are available, an excellent agreement with the experimental values has been achieved. © 2008 The American Physical Society.
 Stanke, M., Kȩdziera, D., Bubin, S., Molski, M., & Adamowicz, L. (2008). Orbitorbit relativistic corrections to the pure vibrational nonBornOppenheimer energies of H2. Journal of Chemical Physics, 128(11).More infoPMID: 18361577;Abstract: We report the derivation of the orbitorbit relativistic correction for calculating pure vibrational states of diatomic molecular systems with electrons within the framework that does not assume the BornOppenheimer (BO) approximation. The correction is calculated as the expectation value of the orbitorbit interaction operator with the nonBO wave function expressed in terms of explicitly correlated Gaussian functions multiplied by even powers of the internuclear distance. With that we can now calculate the complete relativistic correction of the order of α2 (where α=1c). The new algorithm is applied to determine the full set of the rotationless vibrational levels and the corresponding transition frequencies of the H2 molecule. The results are compared with the previous calculations, as well as with the frequencies obtained from the experimental spectra. The comparison shows the need to include corrections higher than second order in α to further improve the agreement between the theory and the experiment. © 2008 American Institute of Physics.
 Stepanian, S. G., Karachevtsev, M. V., Glamazda, A., Karachevtsev, V. A., & Adamowicz, L. (2008). Stacking interaction of cytosine with carbon nanotubes: MP2, DFT and Raman spectroscopy study. Chemical Physics Letters, 459(16), 153158.More infoAbstract: We employed the Raman spectroscopy and the MP2 and DFT (with the MPWB1K, M05, M052X and MPW1B95 functionals) calculations to study the interaction of cytosine with SWCNTs (singlewall carbon nanotubes). The Raman spectra of SWCNT and SWCNTcytosine films showed that the noncovalent interaction results in a downshift of the frequency of the most intensive band of the SWCNT at 1590 cm1 (the Gmode). The interaction energy in the complex formed by cytosine with a fragment of the zigzag(10, 0) SWCNT calculated at the MP2/6311++G(2d,p) level of theory is 62.4 kJ/mol. © 2008 Elsevier B.V. All rights reserved.
 Trzaskowski, B., Leonarski, F., Leś, A., & Adamowicz, L. (2008). Altering the orientation of proteins on selfassembled monolayers: A computational study. Biomacromolecules, 9(11), 32393245.More infoPMID: 18847238;Abstract: A combined computational docking  molecular dynamics study has been performed on a system consisting of cytochrome c protein and alkanethiol selfassembled monolayers of various geometries. The results suggest that the orientation of the protein on the surface may be controlled or altered by means of designing specific structural motifs on the surface. The proposed computational approach may be used as a fast and reliable tool to complement other theoretical and experimental techniques of exploring other protein  surface interfaces. © 2008 American Chemical Society.
 Adamowicz, L., Bubin, S., & Adamowicz, L. . (2007). Calculations of the ground states of BeH and BeH+ without the BornOppenheimer approximation. The Journal of chemical physics, 126(21).More infoNonBornOppenheimer variational calculations employing explicitly correlated Gaussian basis functions have been performed for the ground states of the beryllium monohydride molecule (BeH) and its ion (BeH+), as well as for the beryllium atom (Be) and its ion (Be+). An approach based on the analytical energy gradient calculated with respect to the Gaussian exponential parameters was employed. The calculated energies were used to determine the ionization potential of BeH and the dissociation energies of BeH and BeH+. Also, the generated wave functions were used to compute various expectation values, such as the average interparticle distances and the nucleusnucleus correlation functions.
 Adamowicz, L., Stanke, M., Kedziera, D., Bubin, S., & Adamowicz, L. . (2007). Electron affinity of (7)Li calculated with the inclusion of nuclear motion and relativistic corrections. The Journal of chemical physics, 127(13).More infoExplicitly correlated Gaussian functions have been used to perform very accurate variational calculations for the ground states of (7)Li and (7)Li(). The nuclear motion has been explicitly included in the calculations (i.e., they have been done without assuming the BornOppenheimer (BO) approximation). An approach based on the analytical energy gradient calculated with respect to the Gaussian exponential parameters was employed. This led to a noticeable improvement of the previously determined variational upper bound to the nonrelativistic energy of Li(). The Li energy obtained in the calculations matches those of the most accurate results obtained with Hylleraas functions. The finitemass (nonBO) wave functions were used to calculate the alpha(2) relativistic corrections (alpha=1c). With those corrections and the alpha(3) and alpha(4) corrections taken from Pachucki and Komasa [J. Chem. Phys. 125, 204304 (2006)], the electron affinity (EA) of (7)Li was determined. It agrees very well with the most recent experimental EA.
 Adamowicz, L., Stanke, M., Kedziera, D., Bubin, S., & Adamowicz, L. . (2007). Lowest excitation energy of 9Be. Physical review letters, 99(4).More infoVariational calculations employing explicitly correlated Gaussian functions and explicitly including the nuclear motion [i.e., without assuming the BornOppenheimer (BO) approximation] have been performed to determine the lowest singlet transition energy in the 9Be atom. The nonBO wave functions were used to calculate the alpha2 relativistic corrections (alpha=1/137.035,999,679). With those corrections and with the alpha3 and alpha4 QED corrections determined previously by others, we obtained 54,677.35 cm(1) for the 3(1)S>2(1)S transition energy. This result falls within the error bracket for the experimental transition of 54,677.26(10) cm(1). This is the first time an electronic transition of Be has been calculated from first principles with the experimental accuracy.
 Adamowicz, L., Stanke, M., Kedziera, D., Bubin, S., & Adamowicz, L. . (2007). Relativistic corrections to the nonBornOppenheimer energies of the lowest singlet Rydberg states of 3He and 4He. The Journal of chemical physics, 126(19).More infoIn this work the authors present an approach to calculate the leadingorder relativistic corrections for ground and excited states of helium isotopomers. In the calculations they used variational wave functions expanded in terms of explicitly correlated Gaussians obtained without assuming the BornOppenheimer approximation.
 Adamowicz, L., Volobuyev, M., SaintMartin, H., & Adamowicz, L. . (2007). A molecular dynamics calculations of hole transfer rates in DNA strands. The journal of physical chemistry. B, 111(37).More infoA computational model, which includes both tunneling and thermal hopping mechanisms, has been applied to study the charge transfer in DNA (GC)n and (AT)n strands. The calculations revealed the crucial role played by the A or G NH2group vibrations in the hole transfer in both types of strands. Chargetransfer rates in the two strands have been determined based on the molecular dynamics calculations. They are in good agreement with the available experimental data. The modeling approach used here may be employed in the theoretical study of the charge transfer in natural and artificial DNA strands containing AT and GC pairs.
 Bubin, S., & Adamowicz, L. (2007). Calculations of the ground states of BeH and Be H+ without the BornOppenheimer approximation. Journal of Chemical Physics, 126(21).More infoAbstract: NonBornOppenheimer variational calculations employing explicitly correlated Gaussian basis functions have been performed for the ground states of the beryllium monohydride molecule (BeH) and its ion (Be H+), as well as for the beryllium atom (Be) and its ion (Be+). An approach based on the analytical energy gradient calculated with respect to the Gaussian exponential parameters was employed. The calculated energies were used to determine the ionization potential of BeH and the dissociation energies of BeH and Be H+. Also, the generated wave functions were used to compute various expectation values, such as the average interparticle distances and the nucleusnucleus correlation functions. © 2007 American Institute of Physics.
 Bubin, S., Stanke, M., Kdziera, D., & Adamowicz, L. (2007). Improved calculations of the lowest vibrational transitions in He H+. Physical Review A  Atomic, Molecular, and Optical Physics, 76(2).More infoAbstract: More accurate variational calculations of the lowest three pure vibrational states (v=0,1,2) of the He4 H+ molecular ion have been carried out without assuming the BornOppenheimer approximation. In the calculations we included the complete set of α2 relativistic corrections, i.e., massvelocity, Darwin, spinspin, and orbitorbit. This allowed us to improve the agreement between the theory and the experiment for the vibrational frequencies of the 1→0 and 2→1 transitions as compared to our previous calculations. © 2007 The American Physical Society.
 Bubin, S., Stanke, M., Kdziera, D., & Adamowicz, L. (2007). Relativistic corrections to the groundstate energy of the positronium molecule. Physical Review A  Atomic, Molecular, and Optical Physics, 75(6).More infoAbstract: The leadingorder relativistic corrections to the groundstate energy of the positronium molecule (Ps2) have been computed within the framework of perturbation theory. As the zeroorder wave function we used a highly accurate nonrelativistic variational expansion in terms of 6000 explicitly correlated Gaussians that yielded the lowest variational upper bound for this system to date. We also report some expectation values representing the properties of Ps2. © 2007 The American Physical Society.
 Cafiero, M., & Adamowicz, L. (2007). Nonbornoppenheimer calculations of the ground state of H _{3}. International Journal of Quantum Chemistry, 107(14), 26792686.More infoAbstract: We present quantummechanical calculations for the ground state of the H 3 system performed without the Born  Oppenehimer approximation. In the calculations we use explicitly correlated Gaussian basis functions that explicitly depend on all of the interparticle distances. These basis functions allow us to achieve high accuracy while explicitly describing nucleusnucleus, nucleuselectron, and electronelectron correlation effects. Gaussian basis sets ranging in size from 85 to 950 functions have been optimized using a gradientbased procedure. The issue of defining and extracting the H 3 molecular structure based on the nonBO wave function is also discussed. © 2007 Wiley Periodicals, Inc.
 Chunhui, L. u., Jalbout, A. F., Adamowicz, L., Wang, Y., & Yin, C. (2007). QSRR study for gas and liquid chromatographic retention indices of polyhalogenated biphenyls using two 2D descriptors. Chromatographia, 66(910), 717724.More infoAbstract: Gas chromatographic and liquid chromatographic retention indices of polyhalogenated biphenyl compounds are successfully modeled by using two 2D descriptors of recently introduced MEDV13 and atomtype Estate indices derived directly from the molecular structure. A fivevariable regression equation was generated for the GC retention indices of 54 PHBs with the correlation coefficient (R) and relative standard deviation being 0.9921 and 2.4%. Also, a fivevariable regression equation was generated for the LC retention indices of 56 PHBs with the correlation coefficient (R) and relative standard deviation of 0.9831 and 8.0%. The selected indices have been meaningful by using a molecular fragment analysis tool. Additionally, the leaveoneout crossvalidation method demonstrates that the obtained models are statistically significant and reliable. © 2007 Friedr. Vieweg & Sohn Verlag/GWV Fachverlage GmbH.
 Jalbout, A. F., & Adamowicz, L. (2007). Electron Attachment to DNA Base Complexes. Advances in Quantum Chemistry, 52, 231251.More infoAbstract: In this work we present a review of our groups recent work of our group and that of others related to ab initio studies involving DNA base dimers and complexes. We present recent calculations, as well as some relevant experimental investigations concerning excess electron attachment to these systems. In addition to covalentbound electron attachment, we discuss formation of dipolebound anions, solvated electron systems, and the experimentaltheoretical comparisons that are currently available. Finally, we will discuss possible future avenues of research in this area. © 2007 Elsevier Inc. All rights reserved.
 Jalbout, A. F., Abrell, L. M., Adamowicz, L. ., Polt, R. L., Apponi, A. J., & Ziurys, L. M. (2007). Sugar synthesis from a gasphase formose reaction. Astrobiology, 7(3).More infoPrebiotic possibilities for the synthesis of interstellar ribose through a protic variant of the formose reaction under gasphase conditions were studied in the absence of any known catalyst. The ionmolecule reaction products, diose and triose, were sought by mass spectrometry, and relevant masses were observed. Ab initio calculations were used to evaluate protic formose mechanism possibilities. A bilateral theoretical and experimental effort yielded a physical model for glycoaldehyde generation whereby a hydronium cation can mediate formaldehyde dimerization followed by covalent bond formation leading to diose and water. These results advance the possibility that ionmolecule reactions between formaldehyde (CH(2)O) and H(3)O(+) lead to formose reaction products and inform us about potential sugar formation processes in interstellar space.
 Jalbout, A. F., Abrell, L., Adamowicz, L., Polt, R., Apponi, A. J., & Ziurys, L. M. (2007). Sugar synthesis from a gasphase formose reaction. Astrobiology, 7(3), 433442.More infoPMID: 17630839;Abstract: Prebiotic possibilities for the synthesis of interstellar ribose through a protic variant of the formose reaction under gasphase conditions were studied in the absence of any known catalyst. The ionmolecule reaction products, diose and triose, were sought by mass spectrometry, and relevant masses were observed. Ab initio calculations were used to evaluate protic formose mechanism possibilities. A bilateral theoretical and experimental effort yielded a physical model for glycoaldehyde generation whereby a hydronium cation can mediate formaldehyde dimerization followed by covalent bond formation leading to those and water. These results advance the possibility that ionmolecule reactions between formaldehyde (CH2O) and H3O+ lead to formose reaction products and inform us about potential sugar formation processes in interstellar space. © Mary Ann Liebert, Inc.
 Jalbout, A. F., Abul, M., Trzaskowski, B., & Adamowicz, L. (2007). Formation of glyoxal in hydroxyacetaldehyde and glycine nonenzymatic browning Maillard reaction: A computational study. Food Chemistry, 103(2), 359368.More infoAbstract: Density functional theory (DFT) computations at the standard state on the proposed mechanisms have revealed that glyoxal (Gox) is one of the most possible intermediates in the hydroxyacetaldehyde (Hald) and glycine nonenzymatic browning Maillard reaction under different pH conditions. By following the total mass balance, the gaseous state reaction has been found as more feasible for the formation of Gox than that of the aqueous solution. Hald + deprotonated glycine reaction under basic condition and Hald + unprotonated glycine reaction under neutral condition have been supposed to be more favorable for the production of Gox than Hald + protonated glycine reaction under acidic condition and Hald + glycine zwitterion reaction at the isoelectric point of glycine. Oxidation of Hald to Gox has been found as more plausible in the gaseous state than the aqueous solution. Oxygen has been found as necessary for the production of Gox from Hald. At the standard state, DFT calculations on the proposed mechanisms have evaluated that unprotonated and deprotonated glycine and glycine zwitterion are feasible for liberating NH3, whereas protonated glycine has been found as unfeasible. DFT computations at the standard state on the proposed mechanisms have also revealed that the gaseous state Hald + NH3 reaction is more feasible for the formation of Fald than that of the aqueous solution. Water, which is a byproduct, has found as necessary for the initiation of both of Hald + glycine and Hald + NH3 reactions. © 2006.
 Jalbout, A. F., Castillo, R. D., & Adamowicz, L. (2007). Hydration of excess electrons trapped in charge pockets on molecular surfaces. Chemical Physics Letters, 434(13), 1519.More infoAbstract: In this work we strive to design a novel electron trap located on a molecular surface. The process of electron trapping involves hydration of the trapped electron. Previous calculations on surface electron trapping revealed that clusters of OH groups can form stable hydrogenbonded networks on one side of a hydrocarbon surface (i.e. cyclohexane sheets), while the hydrogen atoms on the opposite side of the surface form pockets of positive charge that can attract extra negative charge. The excess electron density on such surfaces can be further stabilized by interactions with water molecules. Our calculations show that these anionic systems are stable with respect to vertical electron detachment (VDE). © 2006.
 Jalbout, A. F., Castillo, R. D., & Adamowicz, L. (2007). Stabilization of an excess electron on molecular surfaces by a pair of water molecules. Molecular Physics, 105(8), 925932.More infoAbstract: This report presents the results of dimerized water hydration on several hypothetical cyclooctane and cyclohexane molecular surfaces. When these complexes have extensive OH groups on one side of a hydrocarbon surface (i.e. cyclohexane sheets), extended hydrogen bonded networks can form that increase the dipole moment of the system. At the same time, the hydrogen atoms on the opposite side form a pocket of positive charge that can attract excess electrons. In this work two orientations for water dimer hydration on eight molecular surfaces have been studied. All systems have been demonstrated to be stable towards electron detachment.
 Jalbout, A. F., Castillo, R. d., & Adamowicz, L. (2007). Nanoscale molecular surface electron attachment. Chemical Physics Letters, 445(46), 8994.More infoAbstract: In this work we have considered the ability of nanoscale molecular surfaces (around 2 nm in width) in trapping excess electrons. As previously reported, we suggested that molecular surfaces with hydrogen bonding networks (consisting of OH groups) on one side of the surface and hydrogen atoms on the opposite side were capable of forming stable dipolebound anions. The increased dipole moments generated by the OH groups coupled to the partial positive charge of the hydrogen atoms creates charge pockets that are can trap excess electrons. We have extended the size of the surface to study the effect of electron localization on molecular surfaces. © 2007.
 Jalbout, A. F., Leon, A. D., Adamowicz, L., Trzaskowski, B., Chen, E. C., Herder, C., & Chen, E. S. (2007). Theoretical, empirical and experimental electron affinities of SF _{6}: solving the density functional enigma. Journal of Theoretical and Computational Chemistry, 6(4), 747759.More infoAbstract: We postulate that the adiabatic electron affinity of SF 6 is 2.40(10) eV, obtained from the photodetachment onset, 3.2(1) eV and a rearrangement energy of 0.8(1) eV. This eliminates the perceived "SF 6 density functional enigma". All of the experimental electron affinities are evaluated and 14 significantly different values selected to construct empirical anionic Morse potentials. These values are (in eV): 2.40(10), 2.20(10), 2.00(10), 1.80(10), 1.60(10), 1.20(10), 1.07(7), 0.93(10), 0.80(10), 0.63(10), 0.43(10), 0.30(10), 0.10(10), and 0.0(2). Comparisons are made to theoretical calculations and relative bond orders. The calculated electron affinities of SF 6 are: density functional, 1.63.0 eV; ab initio, 1.02.2 eV; semiempirical, 0.12.2eV, and for SF 5 is 0.33.8 eV. Curves calculated using semiempirical, density functional and ab initio procedures are similar to the anionic Morse potentials. This work attempts to correct for differences in the EA calculated for this species. © World Scientific Publishing Company.
 Jalbout, A. F., Pavanello, M., & Adamowicz, L. (2007). The water mediated ring closing in the formose reaction. International Journal of Quantum Chemistry, 107(10), 20242031.More infoAbstract: In this work, we investigate the ring closing mechanisms leading to the formation of formose by highlevel ab initio theoretical calculations. We suggest that a watermediated ring closing mechanism (through the use of H 3O+) is energetically the most favorable pathway for this process. Solvent effects have also been computed and the results further confirm our assertion of the catalytic effect of water in the ringclosing mechanism of the formose reaction. © 2007 Wiley Periodicals, Inc.
 Jalbout, A. F., Trzaskowski, B., Chen, E. C., Chen, E. S., & Adamowicz, L. (2007). Electron affinities, gas phase acidities, and potential energy curves: Benzene. International Journal of Quantum Chemistry, 107(5), 11151125.More infoAbstract: The experimental electron affinities of benzene, Ea(Bz), 0.4 to 4.8 eV, are evaluated. Multiple negative ion states are proposed to account for different electron affinities. The semiempirical procedure known as "configuration interaction or unrestricted orbitals to relate experimental quantities to selfconsistent field values by estimating electron correlation" (CURESEC) has several advantages: (i) supports multiple Ea(Bz), (ii) supports the Ea(phenyl) and the D(CH,Bz), (iii) supports the gas phase acidity of benzene from the latter, (iv) predicts the singlettriplet split for the phenyl anion of 1.2(2) eV, and (v) predicts the existence of an excited quartet state of the benzene anion with an E a(Bz), 2.5(2) eV. Nine ionic Morse curves are calculated from CURESEC properties and experimental data. These are compared with quantum mechanical crossing "c" potentials obtained using a subroutine in commercial software and ab initio and density functional theory (DFT) procedures. Curves are calculated for the proposed quartet state of the benzene anion. © 2006 Wiley Periodicals, Inc.
 Jalbout, A. F., Trzaskowski, B., Xia, Y., Li, Y., Hu, X., Li, H., ElNahas, A., & Adamowicz, L. (2007). Structures, stabilities and tautomerizations of uracil and diphosphouracil tautomers. Chemical Physics, 332(23), 152161.More infoAbstract: A DFT method (B3LYP) and two ab initio methods (MP2 and CCSD(T)) are used to study the stability order and tautomerization processes of all possible uracil and diphosphouracil tautomers. The obtained order of uracil tautomers stability is different from the previous computational investigations. Reliable predictions on the stability order and geometrical structures for the diphosphouracil tautomers are presented for the first time. The dienol tautomers of diphosphouracil are shown to be much more stable than the enolketo and diketo forms, whereas for uracil the diketo form is more favored from the energetical point of view. The comparison of the structures and stability order of the tautomers of the two analogues, allows us to suggest that the interaction between the OH bond and the delocalized π system is crucial for the stabilization of the dienol form of diphosphouracil tautomers. On the other hand, for uracil tautomers only the interaction between the OH bond with the lone pair of the neighboring nitrogen atom plays a major role. Other differences of intramolecular interactions of the two analogues are attributed to the unique bonding property of the phosphorous atom. The energy barriers of eight rotamerization processes and nine proton transfer processes of both analogues are reported. For both systems rotamerization processes are proved to be far more facile than proton transfer processes, since energy barriers for rotamerizations are only several kcal/mol, while energy barriers for proton transfers are up to 50 kcal/mol. The rotamerizations of diphosphouracil rotamers are easier than that of the uracil rotamers, while the proton transfer barriers of uracil tautomers are lower than that of the diphosphouracil tautomers. All these differences are the results of different molecular properties of the two analogues, and also an evidence different intramolecular interactions existed in the two analogues. © 2006.
 Lyakh, D. I., Ivanov, V. V., & Adamowicz, L. (2007). Statespecific multireference completeactivespace coupledcluster approach versus other quantum chemical methods: Dissociation of the N2 molecule. Molecular Physics, 105(10), 13351357.More infoAbstract: A comprehensive comparison of different quantumchemical methods applied to calculate the N2 ground state potential energy curve is presented. In the comparison we highlight the multireference statespecific (MRSS) coupledcluster (CC) approach with the completeactivespace (CAS) reference and with single and double excitations from all reference determinants in the CC operator developed in our group. The method is called CASCCSD. The energy and amplitude equations for the method and the corresponding computer code have been generated using a computerized automative procedure that in the present work was extended to produce a parallel computer code. The complete CASCCSD wave function for N2 includes some selected eightfold excitations in the CC operator. An analysis of the wave function estimates the importance of those excitations at large internuclear separations.
 Pashynska, V., Kosevich, M., Stepanian, S., & Adamowicz, L. (2007). Noncovalent complexes of tetramethylammonium with chlorine anion and 2,5dihydroxybenzoic acid as models of the interaction of quaternary ammonium biologically active compounds with their molecular targets: A theoretical study. Journal of Molecular Structure: THEOCHEM, 815(13), 5562.More infoAbstract: A theoretical study of the interaction of tetramethylammonium cation with chlorine anion and 2,5dihydroxybenzoic acid modeling a carboxylic group and an aromatic ring of side radicals of proteins has been performed by the DFT/B3LYP/631++G** and MP2/631++G** theoretical methods. To evaluate how the solvation affects the interaction, the Polarizable Continuum Models method has been employed. The calculated interaction energies of the noncovalent complexes were compared with the experimental data describing the stability of some noncovalent complexes of a dication of bisquaternary ammonium salt decamethoximun with Cl and 2,5dihydroxybenzoic acid obtained with the massspectrometric technique. © 2007 Elsevier B.V. All rights reserved.
 Pavanello, M., Jalbout, A. F., Trzaskowski, B., & Adamowicz, L. (2007). Fullerene as an electron buffer: Charge transfer in Li@C_{60}. Chemical Physics Letters, 442(46), 339343.More infoAbstract: Electronic properties of neutral and charged Li@C60 complexes have been analyzed. The partial charge of the Li atom in complexes with total charges ranging from +3 to 2 has been studied. Two geometries have been considered: one with Li in the center of the C60 and one with Li 1.2 Å offcenter. The calculations show that the fullerene acts as an electron buffer, with the Li charge remaining +1 regardless of the total charge and the geometry of Li@C60. We explain this with a model that includes the Coulomb interaction between the charged species, and the electron affinities and ionization potentials of Li and C60.
 Slanina, Z., Lee, S., Uhlík, F., Adamowicz, L., & Nagase, S. (2007). Computing relative stabilities of metallofullerenes by Gibbs energy treatments. Theoretical Chemistry Accounts, 117(2), 315322.More infoAbstract: The paper presents computations on endohedral fullerene systems, combining the treatments of quantum chemistry and statistical mechanics. The procedure is illustrated on evaluations of relative concentrations for five isomers of Ca@C72, nine isomers of Ca@C82, four isomers of La@C 82, and two isomers of Sc3N@C80. The results point out the enthalpyentropy interplay in the systems produced under high temperatures. Approaches to description of the encapsulate motions are analyzed and a free encapsulate approximation is suggested for further use. © 2006 SpringerVerlag.
 Slanina, Z., Uhli'k, F., Lee, S., Adamowicz, L., & Nagase, S. (2007). Computations of production yields for Ba@C74 and Yb@C74. Molecular Simulation, 33(7), 563568.More infoAbstract: The paper reports computations for two C74based endohedrals  Ba@C74 and Yb@C74. A set of six C74 cages is considered, namely one cage with isolated pentagons, three isomers with a pentagonpentagon junction, two structures with one pentagonpentagon pair and one heptagon. However, the endohedral based on the cage with isolated pentagons is the most important species in both systems. Special interest is paid to the Gibbs energybased evaluations of the production abundances for different metallofullerenes, for the first time including saturated metal pressures into consideration. The computations show that the saturated metal pressures can substantially modulate the production yields.
 Slanina, Z., Uhlík, F., Lee, S., Adamowicz, L., & Nagase, S. (2007). Calculations on endohedral C _{74} complexes. Journal of Nanoscience and Nanotechnology, 7(45), 13391345.More infoPMID: 17450896;Abstract: While empty C 74 is still a rather elusive fullerene, its endohedral derivatives are well established. The report surveys computations for three of C 74based endohedrals  Ca@C 74, Ba@C 74, and Yb@C 74. A set of six C 74 cages is considered, namely one cage with isolated pentagons, three isomers with a pentagonpentagon junction, two structures with one pair of connected pentagons, and one heptagon. Special interest is paid to the Gibbsenergy evaluations for estimations of the relative populations. Copyright © 2007 American Scientific Publishers All rights reserved.
 Slanina, Z., Uhlík, F., Lee, S., Adamowicz, L., & Nagase, S. (2007). Computed structures and relative stabilities of Be@C_{74}. International Journal of Quantum Chemistry, 107(13), 24942498.More infoAbstract: The paper presents computations for a C74based endohedral  Be@C74. A set of four C74 cages is considered, namely one cage with isolated pentagons and three isomers with a pentagonpentagon junction. The computations are carried out with a newly suggested functional, viz. MPWB1K, which has never been applied to metallofullerenes. Special interest is paid to the Gibbsenergy supported relativestability evaluations. Although the endohedral based on the cage with isolated pentagons is the most important species at lower temperatures, at higher temperatures a cage with pentagonpentagon junction prevails. © 2007 Wiley Periodicals, Inc.
 Slanina, Z., Uhlík, F., Zhao, X., Adamowicz, L., & Nagase, S. (2007). Relative stabilities of Cisomers. Fullerenes Nanotubes and Carbon Nanostructures, 15(3), 195205.More infoAbstract: Relative stabilities of six C74 fullerene cages are evaluated: one species obeying the isolated pentagon rule (IPR), three isomers with a pentagonpentagon junction, two structures with one pentagonpentagon pair and one heptagon. The computations are carried out using the Gibbs energy in a broad temperature interval. It is shown that the IPR cage (D3h symmetry) prevails throughout. As lowlying electronic excited states are possible for the cages, their electronic partition functions are included into consideration. It is argued that for the special conditions of the fullerene synthesis/isolation, the electronic partition function based on the singlet excited states only should better reproduce the experimental population findings. The computations indicate that isolation of other C74 cage, in addition to the IPR isomer, is less likely though not impossible.
 Stanke, M., Kdziera, D., Bubin, S., & Adamowicz, L. (2007). Ionization potential of Be9 calculated including nuclear motion and relativistic corrections. Physical Review A  Atomic, Molecular, and Optical Physics, 75(5).More infoAbstract: Variational calculations employing explicitly correlated Gaussian functions have been performed for the ground states of Be9 and Be+9 including the nuclear motion [i.e., without assuming the BornOppenheimer (BO) approximation]. An approach based on the analytical energy gradient calculated with respect to the Gaussian exponential parameters was employed, leading to energies of the two systems noticeably improved over those found in the recent paper of Pachucki and Komasa. The nonBO wave functions were used to calculate the α2 relativistic corrections (α= e2c). With those corrections and the α3 and α4 corrections taken from Pachucki and Komasa, a new value of the ionization potential (IP) of Be9 was determined. It agrees very well with the most recent experimental IP. © 2007 The American Physical Society.
 Stanke, M., Kdziera, D., Bubin, S., Molski, M., & Adamowicz, L. (2007). Lowest vibrational states of He4 He+3: NonBornOppenheimer calculations. Physical Review A  Atomic, Molecular, and Optical Physics, 76(5).More infoAbstract: Very accurate quantum mechanical calculations of the first five vibrational states of the He4 He+3 molecular ion are reported. The calculations have been performed explicitly including the coupling of the electronic and nuclear motions [i.e., without assuming the BornOppenheimer (BO) approximation]. The nonrelativistic nonBO wave functions were used to calculate the α2 relativistic mass velocity, Darwin, and spinspin interaction corrections. For the lowest vibrational transition, whose experimental energy is established with high precision, the calculated and the experimental results differ by only 0.16 cm1. © 2007 The American Physical Society.
 Stanke, M., Kȩdziera, D., Bubin, S., & Adamowicz, L. (2007). Electron affinity of Li7 calculated with the inclusion of nuclear motion and relativistic corrections. Journal of Chemical Physics, 127(13).More infoAbstract: Explicitly correlated Gaussian functions have been used to perform very accurate variational calculations for the ground states of Li7 and Li7. The nuclear motion has been explicitly included in the calculations (i.e., they have been done without assuming the BornOppenheimer (BO) approximation). An approach based on the analytical energy gradient calculated with respect to the Gaussian exponential parameters was employed. This led to a noticeable improvement of the previously determined variational upper bound to the nonrelativistic energy of Li. The Li energy obtained in the calculations matches those of the most accurate results obtained with Hylleraas functions. The finitemass (nonBO) wave functions were used to calculate the α2 relativistic corrections (α=1c). With those corrections and the α3 and α4 corrections taken from Pachucki and Komasa [J. Chem. Phys. 125, 204304 (2006)], the electron affinity (EA) of Li7 was determined. It agrees very well with the most recent experimental EA. © 2007 American Institute of Physics.
 Stanke, M., Kȩdziera, D., Bubin, S., & Adamowicz, L. (2007). Lowest excitation energy of Be9. Physical Review Letters, 99(4).More infoPMID: 17678358;Abstract: Variational calculations employing explicitly correlated Gaussian functions and explicitly including the nuclear motion [i.e., without assuming the BornOppenheimer (BO) approximation] have been performed to determine the lowest singlet transition energy in the Be9 atom. The nonBO wave functions were used to calculate the α2 relativistic corrections (α=1/137.035999679). With those corrections and with the α3 and α4 QED corrections determined previously by others, we obtained 54677.35cm1 for the 3S1→2S1 transition energy. This result falls within the error bracket for the experimental transition of 54677.26(10)cm1. This is the first time an electronic transition of Be has been calculated from first principles with the experimental accuracy. © 2007 The American Physical Society.
 Stanke, M., Kȩdziera, D., Bubin, S., & Adamowicz, L. (2007). Relativistic corrections to the nonBornOppenheimer energies of the lowest singlet Rydberg states of He3 and He4. Journal of Chemical Physics, 126(19).More infoAbstract: In this work the authors present an approach to calculate the leadingorder relativistic corrections for ground and excited states of helium isotopomers. In the calculations they used variational wave functions expanded in terms of explicitly correlated Gaussians obtained without assuming the BornOppenheimer approximation. © 2007 American Institute of Physics.
 Trzaskowski, B., Jalbout, A. F., & Adamowicz, L. (2007). Functionalization of carbon nanocones by free radicals: A theoretical study. Chemical Physics Letters, 444(46), 314318.More infoAbstract: Methylfunctionalized carbon nanocones of different geometries have been investigated using UFF, AM1, PM3, HF and DFT computational methods. It is shown that carbon atoms at the apices of the nanocones are more reactive the other carbon atoms in the cones. The results of this investigation suggest that it may be possible to selectively functionalize some nanocone carbon atoms by radicals, thus synthesizing novel systems with interesting properties. Additionally, vibrational spectra of methylfunctionalized nanocones have been calculated and analyzed. The results suggest the IR spectra of substituted nanocones should show at which position the substitution occurred. © 2007 Elsevier B.V. All rights reserved.
 Trzaskowski, B., Leś, A., Jalbout, A. F., Adamowicz, L., & Siegbahn, P. E. (2007). Theoretical modeling of the nonenzymatic solvolysis of CMPNeuAc in an acidic environment. Journal of Molecular Structure: THEOCHEM, 820(13), 9097.More infoAbstract: A densityfunctionaltheory investigation of the nonenzymatic solvolysis of the cytydine 5′monophosphate Nacetylneuraminic acid and its derivatives in the acidic environment is presented. The theoretical calculations of the second stage of the reaction mechanism are in agreement with the hypothesis of a dissociative oxocarbeniumlike transition state with proton transfer as a key part of the reaction. The geometries of the transition states of the reactions yielding αmethyl and βmethyl glycosides are essentially different. This study provides new theoretical data that can be helpful in elucidating the mechanism of the carbohydrates hydrolysis as well as other reactions catalyzed by the glycosyltransferases. © 2007 Elsevier B.V. All rights reserved.
 Volobuyev, M., SaintMartin, H., & Adamowicz, L. (2007). A molecular dynamics calculations of hole transfer rates in DNA strands. Journal of Physical Chemistry B, 111(37), 1108311089.More infoPMID: 17722907;Abstract: A computational model, which includes both tunneling and thermal hopping mechanisms, has been applied to study the charge transfer in DNA (GC) n and (AT)n strands. The calculations revealed the crucial role palyed by the A or G NH2group vibrations in the hole transfer in both types of strands. Chargetransfer rates in the two strands have been determined based on the molecular dynamics calculations. They are in good agreement with the available experimental data. The modeling appoach used here may be employed in the theoretical study of the charge transfer in natural and artificial DNA strands containing AT and GC pairs. © 2007 American Chemical Society.
 Witek, H. A., Trzaskowski, B., Małolepsza, E., Morokuma, K., & Adamowicz, L. (2007). Computational study of molecular properties of aggregates of C_{60} and (16, 0) zigzag nanotube. Chemical Physics Letters, 446(13), 8791.More infoAbstract: Molecular properties for two aggregates of C60 and a (16, 0) zigzag nanotube: (a) C60 encapsulated in the nanotube, (b) C60 attached to the outer wall of the nanotube, are studied using the selfconsistent charge densityfunctional tightbinding method with additional dispersion correction. The binding energy for the encapsulated fullerene is 108.3 kcal/mol and for the attached fullerene, only 20.3 kcal/mol. The harmonic vibrational frequencies of the aggregates are found to be almost identical to those obtained for the noninteracting system. Very small extent of the changes upon interaction may pose a challenge to study the aggregated structures using experimental spectroscopic methods. © 2007 Elsevier B.V. All rights reserved.
 Adamowicz, L., Bubin, S., & Adamowicz, L. . (2006). Matrix elements of Nparticle explicitly correlated Gaussian basis functions with complex exponential parameters. The Journal of chemical physics, 124(22).More infoIn this work we present analytical expressions for Hamiltonian matrix elements with spherically symmetric, explicitly correlated Gaussian basis functions with complex exponential parameters for an arbitrary number of particles. The expressions are derived using the formalism of matrix differential calculus. In addition, we present expressions for the energy gradient that includes derivatives of the Hamiltonian integrals with respect to the exponential parameters. The gradient is used in the variational optimization of the parameters. All the expressions are presented in the matrix form suitable for both numerical implementation and theoretical analysis. The energy and gradient formulas have been programmed and used to calculate ground and excited states of the He atom using an approach that does not involve the BornOppenheimer approximation.
 Adamowicz, L., Bubin, S., & Adamowicz, L. . (2006). NonBornOppenheimer variational calculation of the groundstate vibrational spectrum of LiH+. The Journal of chemical physics, 125(6).More infoVery accurate, rigorous, variational, nonBornOppenheimer (nonBO) calculations have been performed for the fully symmetric, bound states of the LiH(+) ion. These states correspond to the ground and excited vibrational states of LiH(+) in the ground (2)Sigma(+) electronic state. The nonBO wave functions of the states have been expanded in terms of spherical Nparticle explicitly correlated Gaussian functions multiplied by even powers of the internuclear distance and 5600 Gaussians were used for each state. The calculations that, to our knowledge, are the most accurate ever performed for a diatomic system with three electrons have yielded six bound states. Average interparticle distances and nucleusnucleus correlation function plots are presented.
 Bubin, S., & Adamowicz, L. (2006). Matrix elements of Nparticle explicitly correlated Gaussian basis functions with complex exponential parameters. Journal of Chemical Physics, 124(22).More infoAbstract: In this work we present analytical expressions for Hamiltonian matrix elements with spherically symmetric, explicitly correlated Gaussian basis functions with complex exponential parameters for an arbitrary number of particles. The expressions are derived using the formalism of matrix differential calculus. In addition, we present expressions for the energy gradient that includes derivatives of the Hamiltonian integrals with respect to the exponential parameters. The gradient is used in the variational optimization of the parameters. All the expressions are presented in the matrix form suitable for both numerical implementation and theoretical analysis. The energy and gradient formulas have been programed and used to calculate ground and excited states of the He atom using an approach that does not involve the BornOppenheimer approximation. © 2006 American Institute of Physics.
 Bubin, S., & Adamowicz, L. (2006). NonBornOppenheimer variational calculation of the groundstate vibrational spectrum of LiH ^{+}. Journal of Chemical Physics, 125(6).More infoAbstract: Very accurate, rigorous, variational, nonBornOppenheimer (nonBO) calculations have been performed for the fully symmetric, bound states of the LiH + ion. These states correspond to the ground and excited vibrational states of LiH + in the ground 2∑ + electronic state. The nonBO wave functions of the states have been expanded in terms of spherical Nparticle explicitly correlated Gaussian functions multiplied by even powers of the internuclear distance and 5600 Gaussians were used for each state. The calculations that, to our knowledge, are the most accurate ever performed for a diatomic system with three electrons have yielded six bound states. Average interparticle distances and nucleusnucleus correlation function plots are presented. © 2006 American Institute of Physics.
 Bubin, S., & Adamowicz, L. (2006). Nonrelativistic variational calculations of the positronium molecule and the positronium hydride. Physical Review A  Atomic, Molecular, and Optical Physics, 74(5).More infoAbstract: Highly accurate groundstate nonrelativistic variational calculations of Ps2, H1 Ps, and H 1 Ps are reported. The calculations have been performed using 5000 explicitly correlated Gaussian basis functions and yielded the lowest variational energy upper bounds for these systems to date. The relative accuracies of the energies obtained are of the order of 4× 1010 a.u. for Ps2 and 2× 109 a.u. for HP. Several expectation values have also been computed for each system, as well as electronpositron annihilation rates. © 2006 The American Physical Society.
 Islam, A. K., Islam, F. N., Iqbal, M. S., Jalbout, A. F., & Adamowicz, L. (2006). Elastic and electronic properties of BeB_{2} in comparison to superconducting MgB_{2} and NbB_{2}. Solid State Communications, 139(6), 315320.More infoAbstract: A study of the structural, elastic and electronic properties of BeB2, a promising diboride in view of the lighter mass of Be, has been made using selfconsistent Density Functional Theory (DFT). The five independent elastic constants of BeB2 are predicted for the first time. The elastic behaviour of the compound is compared with those of the superconducting MgB2 and NbB2. We also evaluate the electronic structure of BeB2 in order to obtain further insight into its surprising difference from the superconducting MgB2. © 2006 Elsevier Ltd. All rights reserved.
 Ivanov, V. V., Adamowicz, L., & Lyakh, D. I. (2006). Dissociation of the fluorine molecule: CASCCSD method and other manyparticle models. International Journal of Quantum Chemistry, 106(14), 28752880.More infoAbstract: The recently developed CAS(n, m)CCSD method is applied to describe the difficult case of the dissociation of the fluorine molecule, F2. The results of the CASCCSD calculations are compared with the results obtained using the CRCCSD method, the RMRCCSD method, and other approaches. We demonstrate that among the methods compared CAS(n, m)CCSD provides the most accurate solution for the whole dissociation path in comparison with the full CI (FCI) results. © 2006 Wiley Periodicals, Inc.
 Ivanov, V. V., Adamowicz, L., & Lyakh, D. I. (2006). Excited states in the multireference statespecific coupledcluster theory with the complete active space reference. Journal of Chemical Physics, 124(18).More infoAbstract: The recently proposed multireference statespecific coupledcluster theory with the complete active space reference has been used to study electronically excited states with different spatial and spin symmetries. The algorithm for the method has been obtained using the computerized approach for automatic generation of coupledcluster diagrams with an arbitrary level of the electronic excitation from a formal reference determinant. The formal reference is also used to generate the genuine reference state in the form of a linear combination of determinants contracted to a configuration with the spin and spatial symmetries of the target state. The naturalorbital expansions of the oneelectron configuration inferaction density matrix allowed us to obtain the most compact orbital space for the expansion of the reference function. We applied our approach in the calculations of singlet and triplet states of different spatial symmetries of the water molecule. The comparisons of the results with values obtained using other manyparticle methods and with the full configuration interaction results demonstrate good ability of the approach to deal with electronic excited states. © 2006 American Institute of Physics.
 Ivanov, V. V., Adamowicz, L., & Lyakh, D. I. (2006). Potential energy surface of the electron excited states in the statespecific multireference coupled cluster theory. Hydrogen fluoride dissociation. Journal of Molecular Structure: THEOCHEM, 768(13), 97101.More infoAbstract: The recently proposed statespecific multireference coupled cluster (SSMRCC) theory with the complete active space (CAS) reference has been tested in calculations of the potential energy surfaces of electronicallyexcited states. The algorithm for the method is derived using the computerbased automated approach for generating the coupled cluster diagrams and the energy and amplitude equations. The active space for the calculation of the target state is constructed using natural orbital expansion obtained from the configuration interaction method with single and double excitations (CISD) oneparticle density matrix. The spinorbitals for the CASCC calculation are obtained using the complete active space self consistent field (CASSCF) method. The numerical example shown concerns several electronically excited states with different spatial and spin symmetries of the hydrogen fluoride molecule. The calculations are performed at several internuclear distances and compared with the results obtained using other methods such as the full configuration interaction (FCI) method, the equationofmotion (EOM) method, and the multireference perturbation theory (MRPT). The comparison shows that the CASCC results are better than the EOM and MRPT results. © 2006 Elsevier B.V. All rights reserved.
 Jalbout, A. F., & Adamowicz, L. (2006). The electron bonded cytosineguanine complex. Chemical Physics Letters, 420(13), 221224.More infoAbstract: In this work, we have performed quantum chemical calculations on a new form of the guaninecytosine anion, in which the excess electron is suspended between the two systems. The vertical detachment energy calculations performed with the MP2 method yield a value of 0.40 eV, showing a stability relative to the addition of this excess electron. The effects of removing the excess electron are also presented, whereby a significant change in the structure occurs. In these geometry optimizations of the neutral structure initiated with geometry of the anion, we obtain a hydrogenbonded dimer with energy lower than the anion.
 Jalbout, A. F., & Adamowicz, L. (2006). The uracil dimer and trimer covalent anions: An ab initio study. Chemical Physics Letters, 420(13), 209214.More infoAbstract: In this work, we present new structures for both the uracil dimer anion (U2) and trimer anion (U3) calculated at the MP2/631++G** level of theory. In these systems, the excess electron is localized at only one uracil unit in a πstate and that molecule shows some offplane distortion leading to higher anion stabilization. In both structures excess electron attachment leads to the formation of a stable covalent anion, with electron affinity values of 0.25, 0.05 eV for U2 and U3, respectively. Experimental implications in relation to the calculated data are also discussed.
 Jalbout, A. F., & Adamowicz, L. (2006). Trapping excess electrons in molecular charge pockets on surfaces. Molecular Physics, 104(19), 31013106.More infoAbstract: This work considers the use of theoretical ab initio calculations to design a novel molecular trap for excess electrons. The basic principle is to develop a model by which these electrons can be stabilized by charge pockets on the surface of molecules. Calculations reveal that systems with OH groups can form stable hydrogenbonded networks on one side of a hydrocarbon surface (i.e. cyclohexane sheets), while the hydrogen atoms on the opposite side of this surface form a pocket of positive charge that attracts the extra negative charge.
 Jalbout, A. F., Adamowicz, L., & Ziurys, L. M. (2006). Conformational topology of ribose: A computational study. Chemical Physics, 328(13), 17.More infoAbstract: This work concerns the theoretical study of the configurational topology of the ribose molecule. MP2/631G** geometry optimizations of the system have yielded several structures corresponding to local minima on the groundstate potential energy surface and the lowest energy configuration was identified. The stability of a few lowest lying conformations has been recalculated at the CCSD(T)/631G** level of theory. © 2006.
 Kedziera, D., Stanke, M., Bubin, S., Barysz, M., & Adamowicz, L. (2006). Darwin and massvelocity relativistic corrections in nonBornOppenheimer variational calculations. Journal of Chemical Physics, 125(8).More infoAbstract: The Pauli approach to account for the massvelocity and Darwin relativistic corrections has been applied to the formalism for quantum mechanical molecular calculations that does not assume the BornOppenheimer (BO) approximation regarding separability of the electronic and nuclear motions in molecular systems. The corrections are determined using the first order perturbation theory and are derived for the nonBO wave function of a diatomic system expressed in terms of explicitly correlated Gaussian functions with premultipliers in the form of even powers of the internuclear distance. As a numerical example we used calculations of the transition energies for pure vibrational states of the HD + ion. © 2006 American Institute of Physics.
 Kosevich, M. V., Boryak, O. A., Orlov, V. V., Shelkovsky, V. S., Chagovets, V. V., Stepanian, S. G., Karachevtsev, V. A., & Adamowicz, L. (2006). Evaluation of the reduction of imidazophenazine dye derivatives under fastatombombardment massspectrometric conditions. Journal of Mass Spectrometry, 41(1), 113123.More infoPMID: 16382482;Abstract: Satellite [M + 2]+• and [M + 3]+ peaks accompanying the common peak of the protonated molecule [M + H]+ that are known to indicate the occurrence of a reduction process were observed in the fast atom bombardment (FAB) mass spectra of imidazophenazine dye derivatives in glycerol matrix. The distribution of the abundances in the [M + nH] + peak group varied noticeably for different derivatives. This indicated different levels of the reduction depending on the different structure variations of the studied molecules. In the search for correlations between the mass spectral pattern and the structural features of the dyes, ab initio HF/631++G** quantum chemical calculations were performed. They revealed that the abundances of the [M + 2]+• and [M + 3] + ions show growth proportional to the decrease of the energy of the lowest unoccupied molecular orbital, i.e. proportional to the increase of the electron affinity of the dye molecule. A method for rapid screening of reductive properties of sets of dye derivatives on the basis of the FAB mass spectral data is discussed. Copyright © 2005 John Wiley & Sons, Ltd.
 Kȩdziera, D., Stanke, M., Bubin, S., Barysz, M., & Adamowicz, L. (2006). Darwin and massvelocity relativistic corrections in the nonBornOppenheimer calculations of pure vibrational states of H _{2}. Journal of Chemical Physics, 125(1).More infoAbstract: The Darwin and massvelocity relativistic corrections have been calculated for all pure vibrational states of the H 2 using the perturbation theory and very accurate variational wave functions obtained without assuming the BornOppenheimer (BO) approximation. Expansions in terms of explicitly correlated Gaussians with premultipliers in the form of even powers of the intranuclear distance were used for the wave functions. With the inclusion of the two relativistic corrections to the nonBO energies the transition energies for the highest states agree more with the experimental results. © 2006 American Institute of Physics.
 Lu, C. H., Jalbout, A. F., Adamowicz, L., Wang, Y., & Yin, C. S. (2006). Prediction of gas chromatographic retention indices of benzene dicarboxylic diesters using novel topological indices. Bulletin of Environmental Contamination and Toxicology, 77(6), 793798.More infoPMID: 17219296;
 Lyakh, D. I., Ivanov, V. V., & Adamowicz, L. (2006). Multireference statespecific coupled cluster approach with the CAS reference: Inserting Be into H_{2}. Theoretical Chemistry Accounts, 116(45), 427433.More infoAbstract: An algorithm for generation of the spinorbital diagrammatic representation, the corresponding algebraical formulas, and the computer code of the coupled cluster (CC) method with an arbitrary level of the electronic excitations developed earlier in our laboratory have been employed to generate the CAS(2,2)CCSD code. CAS(2,2)CCSD is the statespecific, multireference coupled cluster (SSMRCC) approach with single and double excitations based on the CASSCF(2,2) reference wave function. The CAS(2,2)CCSD was used to describe the model process of inserting the Be atom into the H2 molecule. We show that our method performs better than the "fullyblown" SSMRCC approach of Mukherjee and coworkers (J Chem Phys 110:6171, 1999).
 Plokhotnichenko, A. M., Stepanian, S. G., Adamowicz, L., & Karachevtsev, V. A. (2006). Hbonded complexes of 2aminopyrimidineparabenzoquinone in an argon matrix. Fizika Nizkikh Temperatur (Kharkov), 32(2), 201213.More infoAbstract: The IR spectroscopy method was used to study Hbonded complexes of 2aminopyrimidine NH2Py with parabenzoquinon Qu in a lowtemperature argon matrix. The IR absorption spectra of these compounds were obtained in the spectral range 4003600 cm1 at various concentration ratios at T = 11 K. The molar integral absorption coefficients in the bands of stretching vibrations of the free and Hbonded NH2 groups were determined. The quantummechanical calculations of the IR spectra of the NH2Py and Qu molecules and their dimers were performed. Comparison of the experimental and calculated data suggests that the NH2PyQu dimers in the argon matrix have a planar structure with two weak hydrogen bonds NH⋯O and CH⋯N.
 Plokhotnichenko, A. M., Stepanian, S. G., Karachevtsev, V. A., & Adamowicz, L. (2006). Hydrogenbonded complexes of 2aminopyrimidineparabenzoquinone in an argon matrix. Low Temperature Physics, 32(2), 148157.More infoAbstract: The Hbonded complexes of 2aminopyrimidine (NH2 Py) with parabenzoquinone (Qu) in a lowtemperature argon matrix are investigated by the method of IR spectroscopy. The IR absorption spectra in the spectral range 4003600 cm1 are obtained for different concentration ratios of these compounds at a temperature of 11 K. The molar integrated absorption coefficients in the bands of the stretching modes of the free and Hbonded NH2 group are determined. Quantummechanical calculations of the IR spectra of the NH2 Py and Qu molecules and their dimers are carried out. A comparison of the experimental and calculated results permits the conclusion that NH2 PyQu dimers in an argon matrix have a planar structure with two weak hydrogen bonds, NH⋯O and CH⋯N. © 2006 American Institute of Physics.
 Slanina, Z., Lee, S., Uhlík, F., Adamowicz, L., & Nagase, S. (2006). Excited electronic states and relative stabilities of C_{80} isomers. International Journal of Quantum Chemistry, 106(10), 22222228.More infoAbstract: Very high temperatures of fullerene synthesis allow for a significant population of excited electronic states and thus for nonnegligible electronic partition functions. The issue is studied on the C80 isomeric system, as relatively lowlying electronic excited states are known in this case. The set consists of seven isolated pentagonrule (IPR) cages. Computations suggest a species of D5d symmetry as the lowestenergy structure in the set, although in observations a cage of D2 symmetry is more populated. The electronic partition functions are evaluated by means of the ZINDO method. The computed effects of the electronic excited states are still larger than in the previously tested examples. It is shown, however, that for the special conditions of the fullerene synthesis/isolation, the electronic partition function based on the singlet excited states only should better reflect the experimental population findings. The study is complemented with the MP2=FC/631G* evaluation of the separation energy between the D 5d and D2 isomers that places the D2 species ∼2.8 kcal/mol below the D5d cage, i.e., the reversed order than in several previous calculations. © 2006 Wiley Periodicals, Inc.
 Stanke, M., Kȩdziera, D., Molski, M., Bubin, S., Barysz, M., & Adamowicz, L. (2006). Convergence of experiment and theory on the pure vibrational spectrum of HeH+. Physical Review Letters, 96(23).More infoAbstract: Very accurate quantum mechanical calculations of the pure vibrational spectrum of the HeH+ molecular ion are reported and compared with newly obtained pure vibrational transitions extracted from the available experimental data. The calculations are performed without assuming the BornOppenheimer approximation regarding separability of the nuclear and electronic motions and include the first order relativistic massvelocity and Darwin corrections. For the two lowest transitions, whose experimental energies are established with the highest precision, the calculated and the experimental results show very good agreement. © 2006 The American Physical Society.
 Stepanian, S. G., Karachevtsev, V. A., Plokhotnichenko, A. M., Adamowicz, L., & Rao, A. M. (2006). IR spectra of photopolymerized C _{60} films. Experimental and density functional theory study. Journal of Physical Chemistry B, 110(32), 1576915775.More infoPMID: 16898724;Abstract: IR spectra of photopolymerized fullerene films obtained by simultaneous deposition and UV irradiation were measured in the range of 1500450 cm 1. The degree of the polymerization of the G 60 films was estimated to be about 95%. To assist the assignment of the experimental IR spectra of the films, quantum chemical calculations of the equilibrium structures of the C 60 dimers and trimers were performed at the DFT(B3LYP)/ 321G level of theory. Next, IR frequencies and intensities for those structures were calculated. For the fivetrimer structures found in the calculations, the relative stabilities were determined at the B3LYP/431G * and B3LYP/631G* levels and used to select the lowestenergy trimers, which are Trimer A (angle between monomer centers is 90°) and Trimer B (angle between monomer centers is 120°). Next, the IR spectra of the polymerized fullerene films were compared with the calculated frequencies of the lowestenergy dimer and the two lowestenergy trimers. On the basis of this analysis and on the comparison of the film spectra with the IR spectra of the C 60 dimer and trimer spectra obtained by other methods, it was shown that the main components of the films are C 60 dimers and the orthorhombic (O) polymer phase. The tetragonal (T) and rhombohedral (R) polymers, as well as small amounts of monomers, were also found. Although vibrational frequencies of different C 60 phases are similar in most cases, we found several unique spectral features of the C 60 dimer and other polymers that may be used to determine the composition of the polymerized C 60 film. © 2006 American Chemical Society.
 Trzaskowski, B., Jalbout, A. F., & Adamowicz, L. (2006). Molecular dynamics studies of proteinfragment models encapsulated into carbon nanotubes. Chemical Physics Letters, 430(13), 97100.More infoAbstract: In this work, we have explored dynamic properties of two proteinfragment models interacting with singlewalled carbon nanotubes using molecular dynamics. We have considered peptides interacting with the outer walls of nanotubes, encapsulated into nanotubes, as well as covalently bound to nanotubes. The results suggest that the confined space of the nanotube and its interactions with peptides stabilizes the structure of biomacromolecules. On the other hand, covalent linking of peptides to nanotubes may lead to a change in the peptide conformation. The general conclusions derived from this work may be of importance in devising nanotube systems for peptide and drug delivery. © 2006.
 Adamowicz, L., Bednarz, E., Bubin, S., & Adamowicz, L. . (2005). NonBornOppenheimer variational calculations of HT+ bound states with zero angular momentum. The Journal of chemical physics, 122(16).More infoWe report fully nonadiabatic calculations of all rotationless bound states of HT+ molecular ion (t+p+e) carried out in the framework of the variational method. We show that, in all the states, except the two highest ones, the bond in the system can be described as covalent. In the highest two states the bond becomes essentially ionic and HT+ can be described as a T+H+ complex. The wave function of the system was expanded in terms of spherically symmetric, explicitly correlated Gaussian functions with preexponential multipliers consisting of powers of the internuclear distance. Apart from the total energies of the states, we have calculated the expectation values of the tp, te, and pe interparticle distances, their squares, and the nucleusnucleus correlation functions.
 Adamowicz, L., Bubin, S., Bednarz, E., & Adamowicz, L. . (2005). Charge asymmetry in HD+. The Journal of chemical physics, 122(4).More infoExpanding the wave functions of the ground and excited states of HD(+) (or pde) in terms of spherically symmetric explicitly correlated Gaussian functions with preexponential multipliers consisting of powers of the internuclear distance, and using the variational method, we performed very accurate nonadiabatic calculations of all bound states of this system corresponding to the zero total angular momentum quantum number (vibrational states; v=022). The total and the transition energies obtained agree with the best available calculations. For each state we computed the expectation values of the dp, de, and pe interparticle distances. This is the first time these quantities were computed for HD(+) using rigorous nonadiabatic wave functions. While up to the v=20 state some asymmetry is showing in the de and pe distances, for v=21 and v=22 we observe a complete breakdown of the BornOppenheimer approximation and localization of the electron almost entirely at the deuteron.
 Adamowicz, L., Cafiero, M., & Adamowicz, L. . (2005). NonBornOppenheimer molecular structure and oneparticle densities for H2D+. The Journal of chemical physics, 122(18).More infoWe show that the nonadiabatic (nonBornOppenheimer) ground state of a threenuclei system can be effectively calculated with the use of an explicitly correlated Gaussian basis set with floating centers. Sample calculations performed for the H2D+ system with various basis set sizes show good convergence with respect to both the total energy and the expectation values of the internuclear distances (molecular geometry), the distances between the nuclei and the electrons, and between the electrons. We also provide a derivation of the formulas for oneparticle density calculations and some density plots showing the spatial distribution of the H2D+ nuclear and electronic densities.
 Adamowicz, L., Pavanello, M., Bubin, S., Molski, M., & Adamowicz, L. . (2005). NonBornOppenheimer calculations of the pure vibrational spectrum of HeH+. The Journal of chemical physics, 123(10).More infoVery accurate calculations of the pure vibrational spectrum of the HeH(+) ion are reported. The method used does not assume the BornOppenheimer approximation, and the motion of both the electrons and the nuclei are treated on equal footing. In such an approach the vibrational motion cannot be decoupled from the motion of electrons, and thus the pure vibrational states are calculated as the states of the system with zero total angular momentum. The wave functions of the states are expanded in terms of explicitly correlated Gaussian basis functions multipled by even powers of the internuclear distance. The calculations yielded twelve bound states and corresponding eleven transition energies. Those are compared with the pure vibrational transition energies extracted from the experimental rovibrational spectrum.
 Adamowicz, L., Volobuyev, M., & Adamowicz, L. . (2005). Computational model of hole transport in DNA. The journal of physical chemistry. B, 109(2).More infoA computational model based on the molecular dynamics (MD) simulation for the hole transport in DNA has been developed and applied to study hole current in DNA strands consisting of different numbers of GC pairs. The approach is based on the hopping mechanism which is thermally activated. The calculations show that the hole hopping intensifies with the temperature and the transport rate increases in agreement with the experimental evidence. It is also determined that the degree of structural ordering in the DNA strand enhances the hole conductivity and reasons are provided why this may occur.
 Bednarz, E., Bubin, S., & Adamowicz, L. (2005). Integrals for nonBornOppenheimer calculations of molecules with three nuclei. Molecular Physics, 103(68), 11691182.More infoAbstract: Variational molecular calculations without assuming the BornOppenheimer approximation of states corresponding to zero total angular momentum require the use of spherically symmetric basis functions explicitly depending on the electronelectron, electronnucleus, and nucleusnucleus distances. In our calculations, such functions have been the explicitly correlated Gaussians. For molecules with three nuclei, the Gaussians have to be multiplied by powers of all three internuclear distances to describe the highly correlated motion of the nuclei. In this work we have derived formulae for the overlap and the Hamiltonian matrix elements for such basis functions. Implementation of the formulae presents some unconventional numerical difficulties related to maintaining the required precision of the calculation. The implementation problems are discussed. © 2005 Taylor & Francis Group Ltd.
 Bednarz, E., Bubin, S., & Adamowicz, L. (2005). NonBornOppenheimer variational calculations of HT ^{+} bound states with zero angular momentum. Journal of Chemical Physics, 122(16).More infoAbstract: We report fully nonadiabatic calculations of all rotationless bound states of HT+ molecular ion (t+ p+ e) carried out in the framework of the variational method. We show that, in all the states, except the two highest ones, the bond in the system can be described as covalent. In the highest two states the bond becomes essentially ionic and HT+ can be described as a T+ H+ complex. The wave function of the system was expanded in terms of spherically symmetric, explicitly correlated Gaussian functions with preexponential multipliers consisting of powers of the internuclear distance. Apart from the total energies of the states, we have calculated the expectation values of the tp, te, and pe interparticle distances, their squares, and the nucleusnucleus correlation functions. © 2005 American Institute of Physics.
 Borowicz, P., Leś, A., Adamowicz, L., & Waluk, J. (2005). Vibrational spectroscopy of hydroxyheterobiaryls. IRactive modes of [2,2′bipyridyl]3,3′diol. Photochemical and Photobiological Sciences, 4(1), 143148.More infoPMID: 15616705;Abstract: All observed IRactive vibrational modes of [2,2′bipyridyl]3,3′diol (BP(OH)2) and its two isotopomers, BP(OD)2 and BP(OH)2d6 have been assigned on the basis of the comparison of experimental results with predictions obtained from DFT (B3LYP/631 G(d,p)) calculations. The sensitivity of the vibrational transitions to the environment has been studied by comparing the spectra recorded in vapour, different solutions, KBr pellets, and cryogenic argon matrices. The results may be useful for the quantum control of excited state single or double proton transfer in BP(OH)2. © The Royal Society of Chemistry and Owner Societies 2005.
 Bubin, S., & Adamowicz, L. (2005). Nucleusnucleus correlation function in nonBornOppenheimer molecular calculations: Vibrationally excited states of HD^{+}. Chemical Physics Letters, 403(13), 185191.More infoAbstract: In the present work, we studied HD+ (dpe) molecular ion in the framework of variational method without assuming the BornOppenheimer approximation. The nonadiabatic wave function was expanded in terms of explicitly correlated Gaussian basis functions. An algorithm for calculating the nucleusnucleus correlation function (i.e., the probability density of one nucleus in the reference frame where the other one is at the origin) was derived, implemented, and used to depict all bound states of HD+ with zero rotational energy. © 2005 Elsevier B.V. All rights reserved.
 Bubin, S., Adamowicz, L., & Molski, M. (2005). An accurate nonBornOppenheimer calculation of the first purely vibrational transition in LiH molecule. Journal of Chemical Physics, 123(13).More infoAbstract: In this work we study the ground and the first vibrationally excited states of LiH molecule. We performed an extensive nonrelativistic variational calculations of the two states without using the BornOppenheimer approximation. The results are analyzed and compared with the data extracted from recent experiments. The 0←1 transition energy obtained in the calculations converged to a value which is less than a wave number above the transition energy estimated from the available experimental data concerning the LiH rovibrational transitions. We discuss the remaining discrepancy and the procedure used to determine the "experimental" transition frequencies. © 2005 American Institute of Physics.
 Bubin, S., Bednarz, E., & Adamowicz, L. (2005). Charge asymmetry in HD ^{+}. Journal of Chemical Physics, 122(4).More infoAbstract: Expanding the wave functions of the ground and excited states of HD + (or pde) in terms of spherically symmetric explicitly correlated Gaussian functions with preexponential multipliers consisting of powers of the internuclear distance, and using the variational method, we performed very accurate nonadiabatic calculations of all bound states of this system corresponding to the zero total angular momentum quantum number (vibrational states; v=022). The total and the transition energies obtained agree with the best available calculations. For each state we computed the expectation values of the dp, de, and pe interparticle distances. This is the first time these quantities were computed for HD + using rigorous nonadiabatic wave functions. While up to the v=20 state some asymmetry is showing in the de and pe distances, for v=21 and v=22 we observe a complete breakdown of the BornOppenheimer approximation and localization of the electron almost entirely at the deuteron. © 2005 American Institute of Physics.
 Bubin, S., Cafiero, M., & Adamowicz, L. (2005). Nonbornoppenheimer variational calculations of atoms and molecules with explicitly correlated Gaussian basis functions. Advances in Chemical Physics, 131, 377475.
 Cafiero, M., & Adamowicz, L. (2005). NonBornOppenheimer molecular structure and oneparticle densities for H2D+. Journal of Chemical Physics, 122(18).More infoAbstract: We show that the nonadiabatic (nonBornOppenheimer) ground state of a threenuclei system can be effectively calculated with the use of an explicitly correlated Gaussian basis set with floating centers. Sample calculations performed for the H2D+ system with various basis set sizes show good convergence with respect to both the total energy and the expectation values of the internuclear distances (molecular geometry), the distances between the nuclei and the electrons, and between the electrons. We also provide a derivation of the formulas for oneparticle density calculations and some density plots showing the spatial distribution of the H2D+ nuclear and electronic densities. © 2005 American Institute of Physics.
 Hammer, N. I., Compton, R. N., Adamowicz, L., & Stepanian, S. G. (2005). Isotope effects in dipolebound anions of acetone. Physical Review Letters, 94(15).More infoAbstract: Precision measurements using the Rydberg chargeexchange and electric fielddetachment methods find that the dipolebound electron affinity (EA) of acetone (C3H6O) is 55 ± 10 μeV greater than for deuterated acetone (C3D6O). The result agrees well with a theoretical prediction obtained with highlevel electronicstructure and anharmonic vibrational calculations. The dipole moments calculated for the vibrationally averaged structures of C3H6O and C 3D6O show that the isotope effect (2% reduction) on the EA of acetone is mainly due to a slight reduction (0.5%) of the average dipole moment upon deuteration. © 2005 The American Physical Society.
 Ivanov, V. V., Adamowicz, L., & Lyakh, D. I. (2005). Multireference statespecific coupledcluster theory and multiconfigurationality index. BH dissociation. Collection of Czechoslovak Chemical Communications, 70(7), 10171033.More infoAbstract: Multiconfigurationality index calculated for the coupledcluster wave function based on an algorithm developed using a computeraided generation approach is applied to analyze the multireference statespecific coupledcluster method with the CAS reference (i.e. the so called the CAS(n,m)CCSD approach). The numerical results concern dissociation of the BH molecule where at larger displacement from the equilibrium significant quasidegeneracy arises. The analysis shows that the CAS(n,m)CCSD approach performs very well in such a situation.
 Ivanov, V. V., Lyakh, D. I., & Adamowicz, L. (2005). New indices for describing the multiconfigurational nature of the coupled cluster wave function. Molecular Physics, 103(1516), 21312139.More infoAbstract: New cumulative indices which describe the configurational structure and the degree of 'multiconfigurationality' of the coupled cluster (CC) wave function have been proposed and tested on some model systems. The indices calculated for the coupled cluster wave functions generated with the CCSD (single and double excitations) and CCSDT (single, double, and triple excitations) models were compared with the corresponding values obtained with the full configuration interaction (FCI) method. The test calculations have concerned cases where the multireference character of the wave function increases due to bond stretching. © 2005 Taylor & Francis Group Ltd.
 Lyakh, D. I., Ivanov, V. V., & Adamowicz, L. (2005). Automated generation of coupledcluster diagrams: Implementation in the multireference statespecific coupledcluster approach with the completeactivespace reference. Journal of Chemical Physics, 122(2).More infoAbstract: An algorithm for generation of the spinorbital diagrammatic representation, the corresponding algebraical formulas, and the computer code of the coupledcluster (CC) method with an arbitrary level of the electronic excitations has been developed. The method was implemented in the general case as well as for specific application in the statespecific multireference coupledcluster theory (SSMRCC) based on the concept of a "formal reference state." The algorithm was tested in SSMRCC calculations describing dissociation of a single bond and in calculations describing simultaneous dissociation of two single bonds  the problem requiring up to sixparticle excitations in the CC operator.© 2005 American Institute of Physics.
 Mordziński, A., Leś, A., Stepanenko, Y., Rycombel, J., & Adamowicz, L. (2005). S_{0} and S_{1} spectroscopy of jet cooled 9cyano10methylanthracene: The methyl group as a molecular rotor. Journal of Molecular Spectroscopy, 233(1), 98109.More infoAbstract: Jetcooled fluorescence excitation and dispersed fluorescence spectra of 9methylanthracene (MA), 9cyanoanthracene (CA) and 9cyano10methylanthracene (CMA) have been measured. The spectra of MA and CMA near the S 0S1 origin reveal a prominent torsional progression due to the hindered methyl group rotation and its torsional vibration against the aromatic ring frame. Additionally, the laser induced fluorescence LIF excitation spectrum of CMA shows the splitting of many vibrational modes. Observed positions and relative intensities of the methyl internal rotational bands were interpreted in terms of transitions calculated based on the quantum mechanical onedimensional rotor. The lowfrequency vibrational bands were interpreted also with the all electron quantum mechanical calculations within the RHF/631G(d,p), CIS/321G and CIS/631G(d,p) approximations. It is predicted that in the case of MA the eclipsed geometry (one CH in the plane of the ring) is most stable in both S0 and S1 states. Conformation of the methyl group in CMA is suggested to change upon S1 ← S 0 excitation (π/12 phase shift of the methyl group). The predicted energy barrier for methyl group rotation in the S0 state of CMA is considerably higher (72 cm1) than that in the S1 state (22 cm1). Following the present quantum mechanical calculations, the carbon atom of the methyl group belongs to the aromatic plane in the S 0 ground state but it deviates from this plane in the S1 excited state. These in turn suggest that the calculated barrier for methyl group rotation in CMA has a 6fold symmetry in the S0 ground state and roughly a 4fold symmetry in the S1 state. © 2005 Elsevier Inc. All rights reserved.
 Nugent, M. L., & Adamowicz, L. (2005). Stabilization of the adenine covalent anion by microhydration: Theoretical study. Molecular Physics, 103(1112), 14671472.More infoAbstract: The Rydberg electrontransfer spectroscopy (RET) experiment [V. Periquet, A. Moreau, S. Carles, J.P. Schermann, C. Desfrançois, J. Electron. Spectrosc. Relat. Phenom., 106, 141 (2000)] showed that only after solvation by two water molecules the adenine molecule can form a stable covalent anion. Anions of adeninewater complexes containing different numbers of water molecules (A · Wn, n = 14) are investigated in this work with the use of quantum mechanical ab initio calculations. The calculations are used to determine the change of stability of the adenine covalent anion as the number of water molecules in the solvation shell increases. The analysis of the anion electron density is used to describe the localization of the excess electron in the A · Wn complexes. © 2005 Taylor & Francis Group Ltd.
 Pavanello, M., Bubin, S., Molski, M., & Adamowicz, L. (2005). NonBornOppenheimer calculations of the pure vibrational spectrum of HeH ^{+}. Journal of Chemical Physics, 123(10).More infoAbstract: Very accurate calculations of the pure vibrational spectrum of the He H+ ion are reported. The method used does not assume the BornOppenheimer approximation, and the motion of both the electrons and the nuclei are treated on equal footing. In such an approach the vibrational motion cannot be decoupled from the motion of electrons, and thus the pure vibrational states are calculated as the states of the system with zero total angular momentum. The wave functions of the states are expanded in terms of explicitly correlated Gaussian basis functions multipled by even powers of the internuclear distance. The calculations yielded twelve bound states and corresponding eleven transition energies. Those are compared with the pure vibrational transition energies extracted from the experimental rovibrational spectrum. © 2005 American Institute of Physics.
 Slanina, Z., Adamowicz, L., Kobayashi, K., & Nagase, S. (2005). Gibbs energybased treatment of metallofullerenes: Ca@C_{72}, Ca@C_{74}, Ca@C_{82}, and La@C_{82}. Molecular Simulation, 31(23), 7177.More infoAbstract: The paper surveys ongoing computations on endohedral fullerene systems, combining the treatments of quantum chemistry and statistical mechanics. Relative concentrations of four isomers of Ca@C72, six isomers of Ca@C74, nine isomers of Ca@C82, and four isomers of La@C82 are evaluated using the Gibbs energy based on densityfunctional theory (DFT) computations. The results illustrate the enthalpyentropy interplay in the systems produced under high temperatures. Approximations for description of the encapsulate motions are analyzed.
 Slanina, Z., Lee, S., Adamowicz, L., Uhlík, F., & Nagase, S. (2005). Computed structure and energetics of La@C _{60}. International Journal of Quantum Chemistry, 104(2 SPEC. ISS.), 272277.More infoAbstract: Computations are carried out using density functional theory for the endohedral system La@C 60, i.e., the very first metallofullerene observed in gas phase (although not yet isolated in a solid form). The computations offer data on the molecular and electronic structure of the species. In particular, it is shown that the encapsulated atom is not located in the cage center, but is shifted toward the wall. There is a substantial charge transfer from the metal to the cage, at some levels of theory amounting roughly to three electrons. It is shown that there is a relatively large energy stabilization upon encapsulation. However, the entropy term also influences the association equilibrium significantly, so that the standard Gibbsenergy change depends strongly on temperature. The reasons why La@C 60 could not be isolated yet are briefly discussed, as well as a possible relationship to superconductivity. © 2005 Wiley Periodicals, Inc.
 Slanina, Z., Uhlík, F., Adamowicz, L., & Nagase, S. (2005). Computing fullerene encapsulation of nonmetallic molecules: N _{2}@C_{60} and NH_{3}@C_{60}. Molecular Simulation, 31(12), 801806.More infoAbstract: Some endohedral fullerenes have been considered as possible candidate species for molecular memories. Recently, the encapsulation inside the fullerene cages has been extended from atoms to small molecules, for example the nitrogen molecule was placed inside the fullerene cage. The observed N 2@C60 endohedral is computed in the paper together with NH3@C60, which was not yet observed. The computations are based on structural optimizations using densityfunctional theory (DFT) methods. In the optimized structures, the analytical harmonic vibrational analysis was carried out and the encapsulation energetics were evaluated using the second order MøllerPlesset (MP2) perturbation treatment. The lowestenergy structure has the N2 unit oriented towards a pair of parallel pentagons so that the complex exhibits D5d symmetry. At the MP2 level, the encapsulation of N2 into C60 brings a potential energy gain of 9.3 kcal/mol while that for NH3 is 5.2 kcal/mol. The entropy term is also evaluated, yielding the standard Gibbsenergy change at room temperature for the encapsulation of N2 and NH3 of 2.6 and 1.5 kcal/mol, respectively. Some computed structural and vibrational characteristics are also reported. Emerging broader landscape of future applications of such encapsulates in nanoscience and nantechnology is discussed.
 Trzaskowski, B., Leonarski, F., Leś, A., & Adamowicz, L. (2005). Modeling tubulin at interfaces. Immobilization of microtubules on selfassembled monolayers. Journal of Physical Chemistry B, 109(37), 1773417742.More infoPMID: 16853268;Abstract: A theoretical study of protein docking to selfassembled monolayers using a new approach is presented. Docking experiments based on space complementarity implemented in FTDock software were performed for three different proteins: tubulin dimer, cytochrome c, and lysozyme. The proteins were adsorbed on alkanethiol surfaces with different terminating groups and 50 000 best orientations of each protein were analyzed. For all systems three filters based on different chemical and biological approaches were applied. Correctly docked proteins for the cytochrome c and lysozyme systems were found in a list of the first 12 results after applying the geometrical and grouping filter and in a list of the first 3 results after applying the biological filter. We have found that alkanethiol monolayers with odd and even numbers of CH 2 groups have similar properties in terms of interactions with the two proteins. Docking of the tubulin dimer revealed that the orientation favored from the applicational point of view can be found in a list of the first 14 results for monolayers with different terminating groups and that there may be a noticeable difference in tubulin dimer interactions with alkanethiol chains of various length. The results for tubulin dimer docking combined with microtubules ability of reversible assembly suggest that these biological structures may become good candidates to serve as templates for fabrication of nanowires and other nanoscale electronic devices. The new method of theoretical docking presented may be used as a fast and reliable tool complementing other theoretical and experimental techniques of exploring other proteinsurface interfaces. © 2005 American Chemical Society.
 Volobuyev, M., & Adamowicz, L. (2005). Computational model of hole transport in DNA. Journal of Physical Chemistry B, 109(2), 10481054.More infoPMID: 16866478;Abstract: A computational model based on the molecular dynamics (MD) simulation for the hole transport in DNA has been developed and applied to study hole current in DNA strands consisting of different numbers of GC pairs. The approach is based on the hopping mechanism which is thermally activated. The calculations show that the hole hopping intensifies with the temperature and the transport rate increases in agreement with the experimental evidence. It is also determined that the degree of structural ordering in the DNA strand enhances the hole conductivity and reasons are provided why this may occur.
 Adamowicz, L., Bubin, S., & Adamowicz, L. . (2004). NonBornOppenheimer study of positronic molecular systems: e(+)LiH. The Journal of chemical physics, 120(13).More infoVery accurate nonBornOppenheimer variational calculations of the ground state of e(+)LiH have been performed using explicitly correlated Gaussian functions with preexponential factors dependent on powers of the internuclear distance. In order to determine the positron detachment energy of e(+)LiH and the dissociation energy corresponding to the e(+)LiH fragmentation into HPs and Li(+) we also calculated nonBO energies of HPs, LiH, and Li(+). For all the systems the calculations provided the lowest everreported variational upperbounds to the ground state energies. Annihilation rates of HPs and e(+)LiH were also computed. The dissociation energy of e(+)LiH into HPs and Li(+) was determined to be 0.036 548 hartree.
 Adamowicz, L., Bubin, S., & Adamowicz, L. . (2004). Nonrelativistic molecular quantum mechanics without approximations: electron affinities of LiH and LiD. The Journal of chemical physics, 121(13).More infoWe took the complete nonrelativistic Hamiltonians for the LiH and LiH systems, as well as their deuterated isotopomers, we separated the kinetic energy of the center of mass motion from the Hamiltonians, and with the use of the variational method we optimized the groundstate nonadiabatic wave functions for the systems expanding them in terms of nparticle explicitly correlated Gaussian functions. With 3600 functions in the expansions we obtained the lowest ever groundstate energies of LiH, LiD, LiH, and LiD and these values were used to determine LiH and LiD electrons affinities (EAs) yielding 0.330 30 and 0.327 13 eV, respectively. The present are the first highaccuracy ab initio quantum mechanical calculations of the LiH and LiD EAs that do not assume the BornOppenheimer approximation. The obtained EAs fall within the uncertainty brackets of the experimental results.
 Bubin, S., & Adamowicz, L. (2004). NonBornOppenheimer study of positronic molecular systems: e ^{+}LiH. Journal of Chemical Physics, 120(13), 60516055.More infoPMID: 15267488;Abstract: The ground state of e +LiH was discussed to measure nonBornOppenheimer (BO) variational approximations using correlated Gaussian functions with preexponential factors. The nonBO energies of HPs, LiH and Li + were also determined to calculate positron detachment energy of e +LiH and dissociation energy corresponding to e +LiH fragmentation into HPs and Li +. The lowest variational upperbounds to ground state energies were observed for all systems. The e +LiH dissociation energy was found to be 0.036548 hartree and annihilation rates of HPs and e +LiH were also calculated.
 Bubin, S., & Adamowicz, L. (2004). Nonrelativistic molecular quantum mechanics without approximations: Electron affinities of LiH and LiD. Journal of Chemical Physics, 121(13), 62496253.More infoPMID: 15446917;Abstract: A complete nonrelativistic Hamiltonians for the LiH and LiD systems was investigated. The adiabatic electron affinities of LiH and LiD were 0.342±0.012 eV for the former and 0.337±0.012 eV for the latter system. The ground state noadiabatic wave functions were optimized for the systems expanding them in terms of nparticles explicity. With 3600 functions in the expansion the lowest ever ground state energies of LiH, LiD, LiH  and LiH  were obtained.
 Cafiero, M., & Adamowicz, L. (2004). Molecular structure in nonBornOppenheimer quantum mechanics. Chemical Physics Letters, 387(13), 136141.More infoAbstract: We present the first fully nonBornOppenheimer (BO) calculations of the wave function of a molecule with more than two atoms: H3+ and its isotopomers. We also calculate expectation values of the internuclear distances (i.e., the molecular geometry). Since the operators representing the internuclear distances do not commute with the internal nonBO Hamiltonian, the only information that can be acquired from the wave function are the average values of bond distances and for each subset of identical nuclei in the system only one unique interparticle distance can be determined. © 2004 Published by Elsevier B.V.
 Di, W. u., Li, Z., Hao, X., Jalbout, A. F., Adamowicz, L., Li, R., & Sun, C. (2004). An ab initio theoretical prediction: An antiaromatic ring πdihydrogen bond accompanied by two secondary interactions in a "wheel with a pair of pedals" shaped complex FH⋯C_{4}H_{4}⋯HF. Journal of Chemical Physics, 120(3), 13301335.More infoPMID: 15268258;Abstract: Hydrogen bonds have attracted much attention because of their important roles in chemistry and biology. With progress in the study on the hydrogen bond, a number of unusual hydrogen bonds were proposed during the past few decades. This paper aims at exhibiting a kind of new ring πH bond complex with a "wheel with a pair of pedals" shaped (11) structure and revealing the properties of antiaromatic ring π dihydrogen bond and two secondary interactions accompanying it, and enriching the knowledge on novel weak interactions in an exciting part of chemistry.
 Dkhissi, A., Houben, L., Adamowicz, L., & Maes, G. (2004). Spectroscopic studies of proton transfer in hydrogen bonding: Adenine complexed to HCl. Chemical Physics Letters, 387(46), 362366.More infoAbstract: The Hbond interaction of the adenine with HCl is investigated using the combined matrixisolation FTIR and theoretical density functional theory. Theoretical calculations indicate that three closed complexes of the type III (proton transfer or ion pair Hbond), each containing two hydrogen bond, i.e., N1+H⋯Cl⋯HNH; N 3+H⋯Cl⋯HN9 and N7+H⋯Cl⋯HNH, have been obtained. The experimental spectra are consistent with this prediction. © 2004 Elsevier B.V. All rights reserved.
 Jalbout, A. F., Morgado, C. A., & Adamowicz, L. (2004). An excess electron trapped in molecular tweezers: Ab initio study. Chemical Physics Letters, 383(34), 317320.More infoAbstract: In this work, we have used theoretical ab initio calculations to design a molecular trap for an excess electron consisting of two chains of hydrogenbonded HF molecules and a water molecule. A system with a 'tweezers'type shape has been found, where the excess electron is localized between two HF molecules in a ⋯FH·e·HF⋯ bridge. © 2003 Elsevier B.V. All rights reserved.
 Jalbout, A. F., Pichugin, K. Y., & Adamowicz, L. (2004). Interaction of the uracil dipolebound electron with closedshell systems (Ar and N_{2}). Theoretical Chemistry Accounts, 111(26), 358362.More infoAbstract: A very diffuse, but spatially confined, electron trapped in a dipolebound state of a polar molecule provides an excellent target for testing the interaction of a localized electron positioned outside the molecular frame of its host molecule with other atomic or molecular systems. In this work we use ab initio calculations to investigate systems where a dipolebound electron attached to a uracil molecule is interacting with an N2 molecule and an Ar atom. Neither of the two systems forms a stable anion and in the aducts they form with the dipolebound electron the electron becomes suspended between the uracil molecule and Ar or N2. Calculations are performed to determine the vertical electron detachment energies of these anions and to determine the molecular rearrangements occurring when the excess electron is removed from them. © SpringerVerlag 2003.
 Morgado, C. A., Pichugin, K. Y., & Adamowicz, L. (2004). Stabilization of an excess electron on uracil by a pair of HF molecules: Ab initio study. Chemical Physics Letters, 389(13), 1923.More infoAbstract: Two HF molecules can stabilize an excess electron in an uracil anion in a covalent π state or in a diffuse σ state by directly interacting with the electron. The ab initio calculations performed in this work reveal several different configurational isomers of the complex of the uracil anion with the two HF molecules. The systems differ in terms of the structure of the solvation shell around the excess electron. © 2004 Elsevier B.V. All rights reserved.
 Morgado, C. A., Pichugin, K. Y., & Adamowicz, L. (2004). Stabilization of an excess electron on uracil by water. Ab initio study. Physical Chemistry Chemical Physics, 6(10), 27582762.More infoAbstract: Experiments performed on the uracil anion in the gaseous phase indicated that in this system the excess electron is dipolebound. The experiments, however, also indicated that water solvation changes the character of the anion from dipolebound to covalent. In this work we have used ab initio theoretical calculations to investigate the stabilization effect that the attachment of one or two water molecules has on the electron attached to uracil. The calculations concern both dipolebound and covalentlybound electrons and reveal rich configurational isomerism of the complex of the uracil anion with H2O molecules. The systems differ in terms of the structure of the solvation cluster formed around the excess electron.
 Slanina, Z., Uhlík, F., Adamowicz, L., & Lee, S. (2004). Computations of the catalytic effects in the StoneWales fullerene isomerizations: N and CN agents. International Journal of Quantum Chemistry, 99(5 SPEC. ISS.), 634639.More infoAbstract: The StoneWales rearrangement has long been considered as a plausible, albeit hypothetical, mechanism for fullerene annealing and isomerizations. In the view of a recently applied new catalyst, KCN, for incorporation of noble gases in fullerenes, the CN radical is studied here computationally as a possible catalytic species acting in kinetics of the StoneWales fullerene transformation, and a possible role of K + is also investigated. The computations are performed on the PM3optimized bowlshaped model C 34H 12 to which the CN radical is attached by its C or N atom. The activation energies are evaluated at the UPM3, UHF/631G*, UB3LYP / 631G*, ROB3LYP/631G*, ROHF/631G**, and ROB3LYP/631G** levels. However, the reduction of the kinetic barrier owing to the catalyst action is modest so that a free N atom, neutral or charged, still remains a more efficient option. Effects of negatively charged CN species and of K + are also found insufficient. Small amounts of nitrogen are indeed always present during fullerene synthesis, especially from He gas. © 2004 Wiley Periodicals, Inc.
 Slanina, Z., Uhlík, F., Adamowicz, L., Kobayashi, K., & Nagase, S. (2004). Electronic excited states and stabilities of fullerenes: Isomers of C _{78} and Mg@C _{72}. International Journal of Quantum Chemistry, 100(4 SPEC. ISS.), 610616.More infoAbstract: There is one factor in relative stabilities of isomeric fullerenes that has rarely been studied so far  the contribution of excited electronic states. The contribution is dearly quantified by the electronic partition function, supposing the related excitation energies can be evaluated. As temperatures in fullerene synthesis are high, the term should be taken into account. In this article the problem is studied on two isomeric systems. One is the set of five isolated pentagon rule isomers of C 78, relatively well known from experiments. The other is a model set of four isomers of Mg@C 72 (not isolated yet). The electronic excitation energies are computed by the limited configuration interaction (CI) method. In the case of the C 78 isomeric set, the ZINDO semiempirical method is used. Isomers of Mg@C 72 are evaluated by means of the singleexcitation CI or Qsingles (CIS) in the standard LanL2MB and LanL2DZ basis sets  CIS/LanL2MB and CIS/LanL2DZ. It is found that the electronic partition function can cause significant changes in the computed equilibrium relative concentrations of isomers at high temperatures. Metallofullerenes are more likely candidates for such enhanced effects. © 2003 Wiley Periodicals, Inc.
 Slanina, Z., Uhlík, F., Juha, L., Tanabe, K., Adamowicz, L., & Osawa, E. (2004). Computations on C _{84}O: Thermodynamic, kinetic and photochemical stability. Journal of Molecular Structure: THEOCHEM, 684(13), 129133.More infoAbstract: Fullerene oxides were the first observed fullerene derivatives and they have naturally attracted attention of both experiment and theory. C 60O has represented a long standing case of experimenttheory disagreement, and there has been a similar problem with C 60O 2  both cases were explained by kinetic rather than thermodynamic control. In this contribution, we report the first computations of C 84O. The computations are carried out with the PM3 quantumchemical semiempirical method. Thermodynamic stabilities are evaluated for the electronic singlet and triplet states. The triplet states are computed at both restricted openshell HartreeFock (ROHF) and unrestricted openshell HartreeFock levels. The computations focus on the two most abundant C 84 isomers  D 2 and D 2d, and especially deal with additions to their shortest and longest bonds. The D 2/long structure is the lowestenergy species. The PM3/ROHF energy ordering of the C 84O isomers in the first triplet electronic state is exactly the same as in the singlet electronic state and the relative energies are also quite similar. On the other hand, the kinetic stability order is just reversed compared to the thermodynamic order. Hence, for relatively short reaction times the C 84O D 2/short isomer should primarily be formed (in spite of the fact that it should be, thermodynamically, the least stable in the studied set). The computations point out various stability selection rules controlling production of fullerenebased materials. © 2004 Elsevier B.V. All rights reserved.
 Slanina, Z., Zhao, X., Uhlík, F., Lee, S. ., & Adamowicz, L. (2004). Computing enthalpyentropy interplay for isomeric fullerenes. International Journal of Quantum Chemistry, 99(5 SPEC. ISS.), 640653.More infoAbstract: The computations for isolatedpentagonrule (IPR) isomers of C 96 were carried out using four semiempirical quantum chemical methods. The entropy terms were also computed and the relativestability problem was treated in terms of the Gibbs function. It was stated that the absolute stabilities of fullerenes of different dimensions and the absolute values of the heats of formation were required. It was found that the actual entropy and Gibbs free energy changed considerably with pressures on the C 60 and C 70 based on quantum chemical methods.
 Bubin, S., & Adamowicz, L. (2003). Variational calculations of excited states with zero total angular momentum (vibrational spectrum) of H_{2} without use the BornOppenheimer approximation. Journal of Chemical Physics, 118(7), 30793082.More infoAbstract: Variational, nonBornOppenheimer calculations of excited states with zero total angular momentum of H2 molecule were presented. Totally spherically symmetric wave functions were used in the calculations. It was found that very high accuracy in calculations can be achieved by expanding the wave functions in terms of explicitly correlated Gaussian functions.
 Cafiero, M., Adamowicz, L., Duran, M., & Luis, J. M. (2003). Nonadiabatic and BornOppenheimer calculations of the polarizabilites of LiH and LiD. Journal of Molecular Structure: THEOCHEM, 633(23), 113122.More infoAbstract: We present fully nonadiabatic as well as BornOppenheimer calculations of the dipole moments and static polarizabilites of LiH and LiD. The nonadiabatic calculations are done in a basis of explicitly correlated gaussian functions. The electronic BornOppenheimer values are corrected for vibrational contributions using the numerical NumerovCooley method including all orders of anharmonicity. A comparison between the two methods does not suggest any substantial disagreement in absolute values or the isotopic differences of the calculated properties. © 2003 Elsevier B.V. All rights reserved.
 Cafiero, M., Bubin, S., & Adamowicz, L. (2003). NonBornOppenheimer calculations of atoms and molecules. Physical Chemistry Chemical Physics, 5(8), 14911501.More infoAbstract: We review a recent development in highaccuracy nonBornOppenheimer calculations of atomic and molecular systems in a basis of explicitly correlated Gaussian functions. Much of the recent progress in this area is due to the derivation and implementation of analytical gradients of the energy functional with respect to variational linear and nonlinear parameters of the basis functions. This method has been used to obtain atomic and molecular ground and excited state energies and the corresponding wave functions with accuracy that exceeds previous calculations. Further, we have performed the first calculations of nonlinear electrical properties of molecules without the BornOppenheimer approximation for systems with more than one electron. The results for the dipole moments of such systems as HD and LiH agree very well with experiment. After reviewing our nonBornOppenheimer results we will discuss ways this method can be extended to deal with larger molecular systems with and without an external perturbation.
 Jalbout, A. F., Hall, C. S., & Adamowicz, L. (2003). Isomerism of the anion of the indolewater dimer. Ab initio study. Journal of Chemical Physics, 118(23), 1054110546.More infoAbstract: An interesting configuration isomerism of the indole(water) anion was revealed via calculations. Two local minima were predicted to exist on the anion potential energy surface. The first corresponds to a DB anion that was already described in previous calculations, and the second corresponds to a system where the excess electron is suspended between the indole and water molecules. The two peaks were assigned to the dipolebound anion and AISE.
 Jalbout, A. F., Pichugin, K. Y., & Adamowicz, L. (2003). An excess electron connects uracil to glycine: Abinitio study. European Physical Journal D, 26(2), 197200.More infoAbstract: In recent work Gutowski et al. [Eur. Phys. J. D 20, 431 (2002)] reported photoelectronspectroscopy and theoretical study of covalent anion of the uracilglycine complex. In present work we use ab initio calculations to describe an anionic complex of uracil and glycine where the excess electron is localized in a diffuse state between the two monomers. In this system the uracil and glycine molecules are separated by about 4.5 Å and the dipoles of the two monomers point at the excess electron located in the middle of the complex. The calculated fragmentation energy of the anion into a dipolebound uracil anion and a neutral glycine molecule is 1.7 kcal/mol.
 Jalbout, A. F., Pichugin, K. Y., & Adamowicz, L. (2003). An excess electron trapped between thymine and adenine. Ab initio study. Chemical Physics Letters, 376(56), 799805.More infoAbstract: The theoretical ab initio calculations performed in this work revealed two structural isomers of the anion of the canonical pair of nucleic acid bases, thymine and adenine, where the two bases are separated by more than 7 Å but remain connected via an excess electron localized in the middle of the complex. In both systems the two monomers forming the complex point the positive poles of their dipoles at the electron  a configuration highly unstable for the neutral complex. © 2003 Elsevier B.V. All rights reserved.
 Pajak, J., Ramaekers, R., Rospenk, M., Alexandrov, V., Stepanian, S., Adamowicz, L., & Maes, G. (2003). Matrixisolation FTIR studies and theoretical calculations of the vibrational properties of 4methyl2′hydroxy4′ethyloxyazobenzene. Chemical Physics, 286(23), 193204.More infoAbstract: The vibrational spectrum of 4methyl2′hydroxy4′ethyloxyazobenzene (MHEAB) has been studied in lowtemperature argon matrices and in CCl4 solutions. Calculations at the DFT (B3LYP) level of theory with the 631 ++G** basis set, as well as additional IR experiments of deuterated samples and of derivatives without the OH group, have been used to analyze the IR MHEAB spectrum. In the calculations we used the conventional harmonic approach, as well as an approach based on an onedimensional anharmonic model. It has been shown that introduction of the OH group into the compound and formation of the OH ··· N intramolecular hydrogen bond cause an increase of the intensity of the stretching N=N mode, but do not affect the frequencies of the azobridge vibrations. © 2002 Elsevier Science B.V. All rights reserved.
 Ramaekers, R., Dehaen, W., Adamowicz, L., & Maes, G. (2003). Combined matrixisolation FTIR and theoretical study of the intrinsic tautometric, vibrational, and Hbonding properties of N4methoxycytosine. Journal of Physical Chemistry A, 107(11), 17101719.More infoAbstract: The tautomeric, vibrational, and Hbonding characteristics of N4methoxycytosine (N4MC) are investigated by a combined experimental, matrixisolation Fourier transform infrared spectroscopic and computational study. For the theoretical part of the study, the methods RHF, MP2, and DFT (B3LYP) are used with the 631++G** basis set. N4MC occurs in a lowtemperature Ar matrix exclusively in the oxoimino form. Two isomers of this tautomer exist. Using IR intensities of characteristic bands, an approximate value of 2.3 × 102 is obtained for the isomerization constant K1 (anti/syn). Theoretically, two closed Hbonded complexes with water are found for both isomers, i.e., N1H...OH...O=C2 and N3H...OH...O=C2, as well as one less stable open complex, N7...HO and O8...HO, for the syn and anti isomer, respectively. In the experimental FTIR spectra, both closed Hbonded complexes are clearly identified based on characteristic Hbonding absorptions predicted by the calculations. Trace amounts of the open complexes are detected as well. A correlation between the frequency shift of the bonded OH stretching mode and the elongation of the OH distance is presented for NH...OH...X type complexes of N4MC and some comparable compounds.
 Ramaekers, R., Houben, L., Adamowicz, L., & Maes, G. (2003). Correlations between experimental and DFT(B3LYP)/631++G** Hbonding parameters for closed X⋯HO⋯HN (X = N, O or S) Hbonded complexes. Vibrational Spectroscopy, 32(2), 185197.More infoAbstract: The correlations between experimental and DFT(B3LYP)/631++G** calculated parameters for a series of closed Hbonded complexes between a water molecule and a purine or pyrimidine base were investigated. Regular relationship was observed between the calculated Hbond distance and the Hbond angle. It was shown that the DFT(B3LYP)/631++G** method is a very suitable method to predict trends in the parameters related to Hbonding.
 Reva, I. D., Stepanian, S. G., Adamowicz, L., & Fausto, R. (2003). Conformational behavior of cyanoacetic acid: A combined matrix isolation fourier transform infrared spectroscopy and theoretical study. Journal of Physical Chemistry A, 107(33), 63516359.More infoAbstract: This work reports the first experimental study of the cyanoacetic acid (CAA) monomer. Samples of CAA were isolated in lowtemperature argon, krypton, and xenon matrixes and characterized using FTIR spectroscopy. Annealing experiments revealed the presence of different conformers in the matrixes. The direct interconversion of the gauchecis (gc) into the ciscis (cc) conformer was observed upon annealing of matrixes before formation of aggregates. The use of different matrix host gases enabled separation of the observed conformational effects from the sitesplitting of IR absorptions. The assignment of the experimental spectra was based on both the annealing results and the comparison of the experimental data with results of theoretical calculations performed at the B3LYP/augccpVTZ level of theory. The relative energies of the observed conformers, as well as the transition states separating these forms, were calculated at the MP2, MP4, QCISD, and CCSD levels of theory with the augccpVxZ (x = D, T, and Q) basis sets. The planar cc conformer was found to be the lowest energy form of CAA. The second conformer, with a nonplanar backbone, gc, is doubly degenerated by symmetry and was predicted to be 1.3 kJ mol1 (ZPVE corrected CCSD/augccVTZ energies) less stable than the cc form. In matrixes a fraction of the gc conformer undergoes geometrical distortion toward the planar transcis (tc) structure, which in the gaseous phase corresponds to the saddle point between the two mirror gc forms.
 Reva, I. D., Stepanian, S. G., Adamowicz, L., & Fausto, R. (2003). Missing conformers. Comparative study of conformational cooling in cyanoacetic acid and methyl cyanoacetate isolated in low temperature inert gas matrixes. Chemical Physics Letters, 374(56), 631638.More infoAbstract: A comparative conformational study of two related systems, methyl cyanoacetate (MCA) and cyanoacetic acid (CAA), is presented. Ab initio calculations predicted that both systems have two nearly isoenergetic conformers separated by similar low energy barriers (about 3 kJ mol1). In xenon matrixes deposited at temperatures above 40 K for MCA and above 20 K for CAA only one conformer was observed for each of the two systems. However, below those temperatures two MCA and two CAA conformers were trapped into the matrixes. Conformational cooling was found responsible for this behavior. Factors contributing to this effect are discussed. © 2003 Elsevier Science B.V. All rights reserved.
 Schoone, K., Smets, J., Ramaekers, R., Houben, L., Adamowicz, L., & Maes, G. (2003). Correlations between experimental matrixisolation FTIR and DFT(B3LYP) calculated data for isolated 1:1 Hbonded complexes of water and pyridine or imidazole derivatives. Journal of Molecular Structure, 649(12), 6168.More infoAbstract: Relations between selected experimentally observed matrix isolation FTIR and/or DFr(B3LYP)/631++G** predicted parameters are investigated for a series of 1:1 water complexes of pyridine and imidazole derivatives. Regular correlations are observed for: (i) the experimental and calculated OH stretching frequency shifts; (ii) the ratio between the observed and calculated IR stretching frequencies of the water OH group involved in the hydrogen bond and the proton affinity of the hydrogen bonding site (N atom) of the base; (iii) the Hbond interaction energy and the OH stretching frequency shift; (iv) the calculated OH and N⋯H distances and the interaction energy of the Hbond; and (v) the interaction energy of the Hbond and the square root of the intensity enhancement of the OH stretching mode. These correlations are restricted to structurally similar systems (i.e. to the pyridine or the imidazole analogs). These correlations demonstrate that the DFT(B3LYP) method is very suitable to predict trends in the parameters related to the Hbonding. © 2003 Elsevier Science B.V. All rights reserved.
 Slanina, Z., Stobinski, L., Tomasik, P., Lin, H., & Adamowicz, L. (2003). Quantumchemical model evaluations of thermodynamics and kinetics of oxygen atom additions to narrow nanotubes. Journal of Nanoscience and Nanotechnology, 3(12), 193198.More infoPMID: 12908250;Abstract: This paper reports a computational study of oxygen additions to narrow nanotubes, a problem frequently studied with fullerenes. In fact, fullerene oxides were the first observed fullerene derivatives, and they have naturally attracted the attention of both experiment and theory. C 60O had represented a longstanding case of experimenttheory disagreement, and there has been a similar problem with C 60O 2. The disagreement has been explained by kinetic rather than thermodynamic control. In this paper a similar computational approach is applied to narrow nanotubes. Recently, very narrow nanotubes have been observed with a diameter of 5 Å and even with a diameter of 4 Å. It has been supposed that the narrow nanotubes are closed by fragments of small fullerenes like C 36 or C 20. In this report we perform calculations for oxygen additions to such model nanotubes capped by fragments of D 2d C 36, D 4d C 32, and I h C 20 fullerenic cages (though the computational models have to be rather short). The three models have the following carbon contents: C 84, C 80, and C 80. Both thermodynamic enthalpy changes and kinetic activation barriers for oxygen addition to six selected bonds are computed and analyzed. The lowest isomer (thermodynamically the most stable) is never of the 6/6 type, that is, the enthalpically favored structures are produced by oxygen additions to the nanotube tips. Interestingly enough, the lowest energy isomer has, for the D 2d C 36 and D 4d C 32 cases, the lowest kinetic activation barrier as well.
 Slanina, Z., Uhlík, F., & Adamowicz, L. (2003). Computations of model narrow nanotubes closed by fragments of smaller fullerenes and quasifullerenes. Journal of Molecular Graphics and Modelling, 21(6), 517522.More infoPMID: 12676238;Abstract: New type of carbon nanotubes  narrow nanotubes  has recently been observed with diameters of 45Å. It has been postulated that the narrow nanotubes are closed by fullerene fragments of C20 and C36. This paper presents computational results on related model nanotubes with stoichiometries such as C80, C84, C96, C108, or C120. The computations were carried out at the PM3, AM1, SAM1, HF/321G, HF/431G, and B3LYP/631G* levels. Two C36 fullerenes were considered, D6h and D2d. At the PM3 level and with the C84 nanotube stoichiometry, the D2d cage closure gave a lower energy (by 185kcal/mol and a diameter of 5.42Å). There is another possible candidate, a C32 cage with D4d symmetry (two fourmembered rings). At the PM3 level and with the C96 nanotube stoichiometry, the D4d closure (with a diameter of 5.43Å) had energy lower by 210kcal/mol than that of the D6h nanotube closure. On the other hand, fourmembered rings should not play a significant role for narrow nanotubes with a diameter of 4Å, where the dodecahedronrelated closure should be exclusive. Still narrower nanotubes are briefly discussed. © 2003 Elsevier Science Inc. All rights reserved.
 Slanina, Z., Uhlík, F., & Adamowicz, L. (2003). Two isomers of C_{60}F_{48}: Computed interisomeric equilibrium. Fullerenes Nanotubes and Carbon Nanostructures, 11(3), 219226.More infoAbstract: C60F48 has been known to exist in two isomeric forms of D3 and S6 symmetries. However, the quantumchemical calculations have not agreed on their stability order though a nearisoenergetic picture is otherwise always encountered. In order to clarify the situation, the entropy effects are evaluated for synthetic temperatures of about 500K. The entropy evaluations suggest that the D3 isomer should be more stable in the potential energy by 2.052.55 kcal/mol (to which term the ab initio data are closer than the semiempirical ones).
 Slanina, Z., Uhlík, F., Chiang, L. Y., & Adamowicz, L. (2003). Erratum: Structure of hexasulfobutyl fullerenes  A computational study (Fullerenes, Nanotubes, and Carbon Nanostructures (2002) 10:4 (363372)). Fullerenes Nanotubes and Carbon Nanostructures, 11(2), 187.
 Slanina, Z., Uhlík, F., Lee, S., & Adamowicz, L. (2003). Quantumchemical calculations of model systems of interest in fullerenebased superconductivity. Journal of Low Temperature Physics, 131(56), 12591263.More infoAbstract: Quantumchemical computations can supply molecular details useful in understanding fullerencebased superconductivity. This paper deals with computations of three related systems: C60 & Li, C60 & CHCl3 and C60 & CHBr3. Their molecular and electronic structures are discussed, especially charge distributions. Computations are also reported on a model system C660.
 Stepanian, S. G., Jalbout, A. F., Hall, C. S., & Adamowicz, L. (2003). Uraciladenine dimer connected by an excess electron. Journal of Physical Chemistry A, 107(39), 79117914.More infoAbstract: In this work we consider a new form of the uraciladenine dimer anion (AU) where the two monomers are connected by an excess electron suspended between them. An equilibrium structures of the anion was predicted using uncorrelated and correlated ab initio quantummechanical calculations. An AU anion of this kind can be formed in the gas phase by an attachment of an adenine molecule to a dipolebound electron of uracil. Removal of the excess electron from the anion and optimization of the geometry of the neutral cluster starting from the geometry of the anion initially leads to a significant increase of the separation between the bases, but eventually the optimization converges to an Hbonded structure very dissimilar from the anion structure.
 Stepanian, S. G., Karachevtsev, V. A., Glamazda, A. Y., DettlaffWeglikowska, U., & Adamowicz, L. (2003). Combined Raman scattering and ab initio investigation of the interaction between pyrene and carbon SWNT. Molecular Physics, 101(16), 26092614.More infoAbstract: Raman spectra of HiPco SWNT and SWNTpyrene films were measured in the 1601800 cm1 range. Due to the noncovalent interaction between SWNT and pyrene the most intensive component of the SWNT G mode (1590 cm1) is downshifted by 2 cm1 and becomes narrower. Also the intensity of the lowfrequency component of the G mode (1550 cm1) decreases by about 30%. Structures and interaction energies in the complexes of pyrene and zigzag (n, 0) SWNTs [6 ≤ n ≤ 20] were determined at the MP2 level of theory. The BSSEfree geometry optimization of the pyrenezigzag (12,0) SWNT complex converged to a structure with a 1/2staggered conformation and with an intermolecular distance of 3.5 Å. The BSSEfree interaction energy in the complex is 30.8 kJ mol1. Increasing of the nanotube diameter leads to a higher interaction energy. This energy becomes equal to 37.2 kJ mol1 in the case of a planar carbon surface.
 Trzaskowski, B., Les, A., & Adamowicz, L. (2003). Modelling of octahedral manganese II complexes with inorganic ligands: A problem with spinstates. International Journal of Molecular Sciences, 4(89), 503511.More infoAbstract: Quantum mechanical ab initio UHF, MP2, MCSCF and DFT calculations with moderate Gaussian basis sets were performed for (MnX 6, X = H 2O, F , CN , manganese octahedral complexes. The correct spinstate of the complexes was obtained only when the counter ions neutralizing the entire complexes were used in the modelling at the B3LYP level of theory.
 Adamowicz, L., Cafiero, M., & Adamowicz, L. . (2002). NonBornOppenheimer isotope effects on the polarizabilities of H2. Physical review letters, 89(7).More infoWe present nonBornOppenheimer quantummechanical calculations of the behavior of isolated molecules of the H2 isotopomer series in static electric fields. Some conceptual aspects of such calculations are discussed. The values for polarizabilities and hyperpolarizabilities of the H2 isotopomers which we present are the first ever fully nonadiabatic calculated values of these properties.
 Adamowicz, L., Cafiero, M., & Adamowicz, L. . (2002). Nonadiabatic Calculations of the Dipole Moments of LiH and LiD. Physical review letters, 88(3).More infoWe present very highaccuracy fully nonadiabatic calculated values for the dipole moments for the ground states of LiH and LiD. These results were calculated via numerical differentiation of the energy obtained at different electric field strengths. The values for the energy were obtained from variational optimization with analytical gradients of the wave function expanded in a basis of explicitly correlated floating stype Gaussian functions. The values obtained for LiH and LiD, 2.3140 and 2.3088 a.u., are nearly identical to those obtained by experiment.
 Adamowicz, L., Malrieu, J., & Ivanov, V. V. (2002). Alternative multireference statespecific coupled cluster wave functions. International Journal of Molecular Sciences, 3(5), 522549.More infoAbstract: An analysis of alternative expressions of the statespecific (SS) multireference (MR) coupled cluster (CC) wave functions is presented. The approach utilizes the CASSCF (complete active space self consistent field) wave function as the reference. It is designed specifically for calculating excited electronic states. The cluster structure of the CC wave operator and the origin(s) for the cluster expansion(s) are the key features of the development. Test calculations reveal some interesting features of the SMRCC approaches.
 Cafiero, M., & Adamowicz, L. (2002). NonBornOppenheimer calculations of the polarizability of LiH in a basis of explicitly correlated Gaussian functions. Journal of Chemical Physics, 116(13), 55575564.More infoAbstract: NonBornOppenheimer calculations of the electrical static polarizability of lithium hydride molecule were presented. Molecular properties were calculated by expanding the nonBO wave functions in a basis of floating spherical explicitly correlated Gaussians. Energy obtained at different electric field strength was numerically differentiated to calculate the polarizability. Integrals and energy gradients needed to perform the calculations were derived and the result obtained for polarizability was 29.57 au.
 Cafiero, M., & Adamowicz, L. (2002). Nonbornoppenheimer isotope effects on the polarizabilities of H_{2}. Physical Review Letters, 89(7), 073001/1073001/4.More infoAbstract: NonBornOppenheimer quantummechanical calculations of the behavior of isolated molecules of the H2 isotopomer series in static electric fields were carried out. Results were obtained using interpolations with even and oddpower polynomials.
 Cafiero, M., & Adamowicz, L. (2002). Nonadiabatic calculations of the dipole moments of LiH and LiD. Physical Review Letters, 88(3), 033002/1033002/4.More infoAbstract: A method is presented for calculating the dipole moment directly using a variational nonBornOppenheimer wave function expanded in a basis set of floating, stype explicitly correlated Gaussian (FSECG) basis functions. The method may be an excellent one for calculations of molecular properties in general.
 Galetich, I., Stapanian, S. G., Shelkovsky, V., Kosevich, M., & Adamowicz, L. (2002). Mass spectrometric and ab initio study of the interaction between 9methylguanine and amino acid amide group. Molecular Physics, 100(23), 36493659.More infoAbstract: The temperature dependent field ionization mass spectrometry method combined with ab initio calculations was used to determine the interaction energies and the structures of 9methylguanineacrylamide dimers. Acrylamide mimics the side chain amide group of the natural amino acids asparagine and glutamine. The experimental enthalpy of the dimer formation derived from the van't Hoff plot is 59.5 ± 3.8 kJ mol1. The value is higher than interaction energies between acrylamide and other nucleic acid bases which were determined to be 57.0 for 1methylcytosine, 52.0 for 9methyladenine, and 40.6 kJ mol1 for 1methyluracil. In total, eight hydrogen bonded dimers formed by the three lowest energy 9methylguanine tautomers and acrylamide were found in the quantum chemical calculations performed at the DFT/B3LYP/631++G** and MP2/631++G** levels of theory. The relative stability and the interaction energies of the dimers were calculated accounting for the basis set superposition error and the zeropoint vibrational energy correction. The lowest energy dimer found in the calculations is formed by acrylamide (Ac) with the keto tautomer of 9methylguanine (Gk). It is stabilized by two intermolecular H bonds, C6=O(Gk) ⋯ HN(Ac) and N1H(Gk) ⋯ O(Ac), and it is more stable than the second lowest energy dimer by ≈ 25 kJ mol1. The calculated interaction energies of the lowest energy 9methylguanineacrylamide dimer are 65.0 kJ mol1 and 67.7 kJ mol1 at the MP2 and DFT levels of theory, respectively. The experimental enthalpy of the dimer formation is in good agreement with both the calculated interaction energies of the GkAc dimer and much higher than the interaction energies calculated for all other 9methylguanineacrylamide dimers. This proved that only one dimer was present in the experimental samples. To verify whether acrylamide is a good model of the amino acidamide group, we performed direct calculations of the 9methylguanineglutamine dimers at the same levels of theory as used for the complexes involving acrylamide. The interaction energies found for the lowest energy 9methylguanineglutamine dimer are 65.1 kJ mol1 (MP2/631++G**) and 66.2 kJ mol1 (DFT/B3LYP/631++G**) and these values are very close (within 0.5 kJ mol1) to the interaction energies obtained for the 9methylguanineacrylamide dimers.
 Gutowski, M., Hall, C. S., Adamowicz, L., Hendricks, J. H., Clercq, H. D., Lyapustina, S. A., Nilles, J. M., Xu, S. ., & Bowen Jr., K. H. (2002). Solvated electrons in very small clusters of polar molecules: (HF)_{3}^{}. Physical Review Letters, 88(14), 143001/1143001/4.More infoAbstract: The coexistence of dbe and se in as small a cluster as (HF)3 is illustrated. The stability of these anions with respect to the neutral cluster results not only from the excess electron binding energy but also from favorable entropic effects, which reflect the greater "floppiness" of the anionic structures.
 Hall, C. S., & Adamowicz, L. (2002). Interaction of the uracil dipolebound electron with noble gas atoms. Molecular Physics, 100(21), 34693473.More infoAbstract: Very diffuse, but localized, electrons trapped in dipolebound states of polar polyatomic molecules may provide excellent targets for testing electronmolecule interactions. Ab initio calculations are used to investigate systems where a dipolebound electron attached to a uracil molecule is interacting with noble gas atoms (He and Ne) and forming very weakly bound adducts. In these adducts, the noble gas atoms are separated from the uracil molecule by considerable distances, and the excess electron is suspended between the uracil molecule and the noble gas atom. Calculations are performed to determine the vertical electron detachment energies of these systems and to determine what happens when the excess electrons are removed from them.
 Hall, C. S., & Adamowicz, L. (2002). Two uracil molecules connected by an excess electron. Journal of Physical Chemistry A, 106(25), 60996101.More infoAbstract: In this work we considered a new form of the uracil dimer anion (U2). In this system the excess electron is suspended between two uracil molecules and provides sufficient bonding between them to stabilize the dimer in a local minimum on its potential energy surface. Ab initio calculations are employed to describe the system. Anions of the nucleic acid bases of this kind can be formed in two steps. First a dipolebound anion is formed by a molecule of one of the bases. Then a molecule of the second base attaches to the diffuse dipolebound electron at the opposite side with respect to the first base. Removal of the excess electron leads to a significant change of the structure of the complex. Its geometry optimization initiated with the anion geometry converges to a conventional hydrogenbonded dimer with a total energy lower than the energy of the anion. Thus the two uracils connected by a suspended excess electron in the middle are a metastable system.
 Jalbout, A. F., & Adamowicz, L. (2002). Anionaromatic molecule complex. Ab initio study of the benzene·O_{2} anion. Journal of Chemical Physics, 116(22), 96729676.More infoAbstract: Ab initio calculations were carried out for the O2 benzene complex and its anions to determine their adiabatic electron affinity and structures. Differences between the molecular and electronic structures of the anion and the neutral complex, and between the free and complexed O2 were revealed by the calculations. Analysis of the wave function of [O2·benzene] showed delocalization of excess electrons from O2 to the region of the σ electrons in benzene.
 Jalbout, A. F., & Adamowicz, L. (2002). Cluster size effects upon stability of adeninemethanolanions. Theoretical study. Journal of Molecular Structure, 605(1), 93101.More infoAbstract: Rydberg electron transfer spectroscopy (RET) experiments [J. Electr. Spectr. Relat. Phenom. 106 (2000) 141] showed that only after being solvated with three methanol molecules the adenine molecule can form a stable covalent anion. Anions of adeninemethanol complexes (A·Mn, n = 13) are investigated in the present work with the use of quantum mechanical calculations. These complexes are good models to study how solvation affects the stability on the adenine anion. It is shown that among several hydrogenbonded configurations of the A·Mn, n = 13, complexes there are systems which can form stable dipolebound (DB) anions. These systems, however, are not the lowest energy structures. The stability of the covalent anion of A·M3 is also investigated. © 2002 Elsevier Science B.V. All rights reserved.
 Jalbout, A. F., HallBlack, C. S., & Adamowicz, L. (2002). An excess electron suspended between two polar molecules. Ab initio study of uracil · H_{2}O and uracil · HF anions. Chemical Physics Letters, 354(12), 128133.More infoAbstract: Metastable anions of uracil complexes with water and hydrogen fluoride where the excess electron is suspended between the two monomers have been investigated using ab initio calculations. Anions of this class can be formed by attachment of a polar molecule to a dipolebound excess electron of the uracil anion. This provides a bonding effect sufficient to stabilize the dimer in a local minimum of the potential energy surface. Removal of the excess electron leads to a significant change of the structure of the complex which rearranges and forms a conventional hydrogenbonded dimer. © 2002 Published by Elsevier Science B.V.
 Ramaekers, R., Adamowicz, L., & Maes, G. (2002). Tautomery and Hbonding characteristics of 2aminopurine: A combined experimental and theoretical study. European Physical Journal D, 20(3), 375388.More infoAbstract: An experimental and theoretical RHF, MP2 and DFT/631++G** study is described of the matrix FTIR spectra of monomer 2aminopurine and Hbonded complexes of 2aminopurine with water. 2aminopurine occurs in Ar predominantly as the aminoN9H tautomer, but small amounts of the aminoNTH tautomer are also present. An approximate KT value for this tautomeric equilibrium is found to be 0.016 (RHF) and 0.015 (DFT) using the infrared intensity measurement. Four Hbonded complexes of the abundant aminoN9H form with water are detected in the experimental FTIR spectrum by their characteristic predicted absorptions, i.e. the three closed complexes N3⋯ H O⋯HN9, N1⋯ HO⋯ H NH, N3⋯ HO⋯ HNH and the open complex NT⋯ H OH. From the experimental results, the proton affinity of the N7 atom in 2aminopurine can be estimated. The dependence of the Hbond strength on the Hbond linearity is demonstrated by a correlation between the N⋯ H distance and the N⋯ H O angle in closed N⋯ HO⋯ HN complexes.
 Ramaekers, R., Dkhissi, A., Adamowicz, L., & Maes, G. (2002). Matrixisolation FTIR study and theoretical calculations of the hydrogenbond interaction of hypoxanthine with H_{2}O. Journal of Physical Chemistry A, 106(18), 45024512.More infoAbstract: The Hbonding interaction between the oxopurine base hypoxanthine and water is investigated using matrixisolation FTIR spectroscopy combined with theoretical density functional theory and ab initio methods. For vibrational frequency predictions, only the DFT/B3LYP method is used, while for the prediction of relative energies and Hbond interaction energies of the complexes, four different theoretical methods are compared, i.e., RHF//RHF, MP2//RHF, DFT//DFT, and MP2//DFT. The oxoN1HN7H, the oxoN1HN9H, and the hydroxyN9H (rotamer with the hydroxyH atom pointed toward the N1 atom) are found to be the three most stable tautomeric forms of the free base. Different, stable complexes between these three tautomers and water have been predicted theoretically. The experimental FTIR spectra agree well with this prediction, and most of the characteristic spectral data for the four most stable closed complexes, N7H···OH···O=C6 and N1H···OH···O=C6 of the O17 tautomer and N1H···OH···O=C6 an N9H···OH···N3 of the O19 tautomer, have been identified in the spectrum. The closed complexes with two Hbonds are the most stable systems due to the Hbond cooperative effect. The obtained results allow to analyze the important relation between cooperativity and (non)linearity of NH···OHbonded systems.
 Rostkowska, H., Lapinski, L., Nowak, M. J., & Adamowicz, L. (2002). Normal mode analysis of the vibrational spectrum of tropolone  A molecule with sevenmembered ring. International Journal of Quantum Chemistry, 90(3), 11631173.More infoAbstract: Infrared spectra of tropolone (2hydroxy2,4,6cycloheptatriene1one) isolated in Ar and N2 matrices are compared with the spectra theoretically calculated at the DFT (density functional theory) Becke, Lee, Yang, Paar (B3LYP)/631++G** and MøllerPlesset of the second order (MP2)/631+G* levels. The good agreement between experimental spectrum and DFT results allowed assignment of the bands observed in the IR spectrum to the theoretically predicted normal modes described in terms of potential energy distribution (PED). Internal coordinates describing inplane and outofplane deformations of the sevenmembered ring of D7h symmetry (and for eightmembered ring of D8h symmetry) were designed for the purpose of normal mode analysis. © 2002 Wiley Periodicals, Inc. Int. J. Quantum Chem. 90.
 Shelkovsky, V. S., Stepanian, S. G., Galetich, I. K., Kosevich, M. V., & Adamowicz, L. (2002). Modeling of recognition sites of nucleic acid bases and amide side chains of amino acids. Combination of experimental and theoretical approaches. European Physical Journal D, 20(3), 421430.More infoAbstract: A combined experimentaltheoretical approach to modeling of building blocks of recognition complexes formed by nucleic acid bases and the aminoacids sidechain amino group is reviewed. The approach includes the temperature dependent fieldionization mass spectrometry and ab initio quantum chemical calculations. The mass spectrometric technique allows determination of interaction enthalpies of biomolecules in the gas phase, and the results it produces are directly comparable to the results obtained through theoretical modeling. In our works we have analyzed both thermodynamic and structural aspects of the recognition complexes of four canonical nucleic acid bases and acrylamide, which models the side chain of asparagine and glutamine. It has been shown that all bases can interact with amide group of the amino acids via their WatsonCrick sites when being incorporated into a single strand DNA or RNA. Stability of the complexes studied, expressed as Δ?H (kJ mole1) decreases as: m9Gua (59.5) ≥ m1Cyt (57.0) ≥ m9Ade (52.0) ≫ m1Ura (40.6). We have determined that in the double stranded DNA only purine bases can be recognized.
 Slanina, Z., Uhlík, F., Chiang, L. Y., & Adamowicz, L. (2002). Structure of hexasulfobutyl fullerenes: A computational study. Fullerenes Nanotubes and Carbon Nanostructures, 10(4), 363372.More infoAbstract: Quantumchemical computations are reported for hexasulfobutyl fullerenes C60((CH2)4SO3H)6 (FC4S). FC4S represent novel watersoluble fullerene derivatives with an enhanced radical scavenging activity and also possible potential for materials science. The computations have been carried out with the standard version of the PM3 semiempirical quantumchemical method. It is found out that the lowest among the computed eight FC4S species has the structural pattern already observed for C60Cl6. However, a largerscale search is at present prevented by the demands on computational resources.
 Cafiero, M., & Adamowicz, L. (2001). Analytical gradients for Singer's multicenter nelectron explicitly correlated Gaussians. International Journal of Quantum Chemistry, 82(4), 151159.More infoAbstract: Analytical gradients for Singer's basis of nelectron multicenter explicitly correlated Gaussian functions are derived and implemented to variationally optimize the energy and wave function of molecular systems within the BornOppenheimer approximation. Wave functions are optimized with respect to (1/2n(n + 1) + 3n) nonlinear variational parameters and one linear coefficient per term in the basis set. Preliminary results for the ground states of H3+ and H3 suggest that the method can be more flexible and can achieve lower energies than previously reported calculations. © 2001 John Wiley & Sons, Inc.
 Cafiero, M., & Adamowicz, L. (2001). Simultaneous optimization of molecular geometry and the wave function in a basis of Singer's nelectron explicitly correlated Gaussians. Chemical Physics Letters, 335(56), 404408.More infoAbstract: Analytical gradients of the BornOppenheimer variational energy of a molecular system with respect to nuclear coordinates are derived and implemented to optimize the molecular geometry in a basis of Singer's explicitly correlated Gaussian functions. The 3N6 geometry variables are optimized simultaneously with the linear and nonlinear variational parameters of the wave function. The method can be used for global minimization, transition structure prediction, and following reaction paths in problems that require very accurate wave functions. Test results on hydrogen clusters with two and three atoms are presented as an illustration of the method.
 Carles, S., Desfrançois, C., Schermann, J. P., Jalbout, A. F., & Adamowicz, L. (2001). Dipolebound anion of 1,3butanediol. Ab initio and Rydberg electron transfer spectroscopy study. Chemical Physics Letters, 334(46), 374380.More infoAbstract: Rydberg electron transfer spectroscopy (RET) has been used to determine the dipolebound (DB) electron affinity (EA) of 1,3butanediol and a single spectral feature was obtained corresponding to the EA value of 8±1 meV. A series of highlevel ab initio calculations were also carried out for this system. They revealed an interesting configuration landscape of the 1,3butanediol DB anion with four minima corresponding to different configurations. All four configurations can be responsible for the single RET spectral peak. © 2001 Elsevier Science B.V.
 Jalbout, A. F., & Adamowicz, L. (2001). Dipolebound anions of adeninewater clusters. ab initio study. Journal of Physical Chemistry A, 105(6), 10331038.More infoAbstract: An interesting configuration landscape has been predicted for dipolebound (DB) anions of [adenine(water),N] complexes for N = 1,2, and 3 using ab initio calculations. In these systems, the excess electron is bonded through the interaction with the dipole moment of the cluster. Since the DB electron affinities of adenine and water are practically zero, only if the water(s) dipole(s) aligns with the adenine dipole, the adenine ability to dipoleattach an excess electron can be sufficiently amplified to enable detection. This effect is investigated with the calculations. © 2001 American Chemical Society.
 Jalbout, A. F., & Adamowicz, L. (2001). Dipolebound anions to adenineimidazole complex. Ab initio study. Journal of Physical Chemistry A, 105(6), 10711073.More infoAbstract: An interesting configurational change has been predicted for the adenineimidazole complex upon dipolebound attachment of an excess electron to this system. The presence of the electron reverses the stability order of the two lowest energy configurations of the complex. One should be able to detect this effect experimentally using Rydberg electrontransfer spectroscopy. © 2001 American Chemical Society.
 Jalbout, A. F., Smets, J., & Adamowicz, L. (2001). Anions of the hydrogenbonded thymine dimer: Ab initio study. Chemical Physics, 273(1), 5158.More infoAbstract: Theoretical calculations have been performed to determine the ability of the hydrogenbonded thymine dimer to form stable anions. The major conclusions of this work are: (i) three of the hydrogenbonded conformers of the thymine dimer can form stable dipolebound anions with excess electrons; (ii) thymine dimer can form a covalent anion that has a structure dissimilar from the structures of the neutral dimer; (iii) in the covalent thyminedimer anion the excess electron is localized in a πorbital on one of the thymine molecules and this molecule shows an outofplane distortion; (iv) the covalent thyminedimer anion is stable with respect to an adiabatic electron detachment. © 2001 Elsevier Science B.V. All rights reserved.
 Lee, J. M., & Adamowicz, L. (2001). Ab initio calculations of electronically excited states of cyanosubstituted polyacetylene cations. Spectrochimica Acta  Part A: Molecular and Biomolecular Spectroscopy, 57(4), 897906.More infoPMID: 11345262;Abstract: The transition energies for the lowest energy π→π* electronic excitations are calculated with the complete active space selfconsistent field method (CASSCF) and with the complete active space secondorder perturbation theory method (CASPT2) for the linear cyanosubstituted polyacetylene cations, HCnCN+, n = 411, and NCCnCN+, n = 210. These systems are models for an important class of interstellar species. We demonstrate the utility of the theoretical calculations in assigning the experimental spectra. © 2001 Elsevier Science B.V.
 Ramaekers, R., Maes, G., Adamowicz, L., & Dkhissi, A. (2001). Matrixisolation FTIR study and theoretical calculations of the vibrational, tautomeric and Hbonding properties of hypoxanthine. Journal of Molecular Structure, 560(13), 205221.More infoAbstract: The neutral compound hypoxanthine is investigated using the technique of matrixisolation FTIR spectroscopy combined with density functional theory (DFT) and ab initio methods. Two theoretical methods (RHF and DFT/B3LYP) are compared for vibrational frequency prediction, and four methods (RHF//RHF, MP2//RHF, DFT//DFT and MP2//DFT) for prediction of the relative energies of the tautomers and the interaction energies of the complexes. All the possible tautomeric forms have been considered theoretically, and the results indicate that two oxo forms (O17 and O19) and one hydroxy form (H9rl) are the three most stable forms. The experimental FTIR spectra are consistent with this prediction, and nearly all the characteristic spectral features of these forms have been identified in the spectrum. A theoretical study of the Hbonded complexes of these three tautomers with water is also performed. Several structures have been found for each form and the results demonstrate that the closed complexes with two Hbonds are the most stable systems due to the Hbond cooperative effect. © 2001 Elsevier Science B.V.
 Reva, I. D., Stepanian, S. G., Adamowicz, L., & Fausto, R. (2001). Combined FTIR matrix isolation and Ab initio studies of pyruvic acid: Proof for existence of the second conformer. Journal of Physical Chemistry A, 105(19), 47734780.More infoAbstract: The molecular structure of pyruvic acid was investigated by matrix isolation FTIR spectroscopy, density functional theory (DFT), and ab initio calculations performed at the RHF, MP2, MP4(SDQ), and CCSD(T) levels of theory with the augccpVDZ basis set. In these calculations, the geometries of the three lowest energy conformers of pyruvic acid were fully optimized at the DFT/B3LYP/augccpVDZ and MP2/augccpVDZ levels. Additionally, the relative energies of the conformers were calculated at the MP4, CCSD, and CCSD(T) levels. Harmonic frequencies and IR intensities were then calculated for these three conformers and were used to account for the zero point vibrational energy corrections and to assist the assignment of the observed bands to the different forms. We found that two conformers are present in the Ar matrix, and both forms exhibit a planar framework with the carbonyl bonds in a trans arrangement but differ in the orientation of the hydroxyl hydrogen. By varying the temperature of the pyruvic acid vapor prior to matrix deposition we were able to separate the bands due to the two conformers and measure their enthalpy difference. The spectral signature of the second pyruvic acid conformer has been identified for the first time. Experimental enthalpy difference between the two most stable conformers was found to be 8.7(±15%) kJ mol1, which is in good agreement with the theoretical result.
 Rostkowska, H., Nowak, M. J., Lapinski, L., & Adamowicz, L. (2001). IR spectral and theoretical characterization of intramolecular hydrogen bonds closing fivemembered rings. Physical Chemistry Chemical Physics, 3(15), 30123017.More infoAbstract: A series of molecules with a common structural feature of a fivemembered quasiring closed by an intramolecular hydrogen bond OH··· O=C interaction has been studied. In the series the compounds differ by the size of the mainring which varies from 4 to 7 atoms. The molecules were isolated in lowtemperature matrices and investigated by IR spectroscopy. The strength of the OH··· O=C interaction, depending on the size of the main ring, was spectroscopically revealed by the position of the infrared bands due to the stretching and torsional vibrations of the OH group. All the intermediate cases from very weak hydrogenbondlike interaction to medium strength hydrogen bonding were observed, indicating that the strict boundary between the cases "hydrogen bond" and "no hydrogen bond" does not exist. This experimental conclusion is supported by theoretical calculations, performed at HF/631++G**, MP2/631 + G* and DFT(B3LYP)/631 ++G** levels.
 Scheu, C. E., Kinghorn, D. B., & Adamowicz, L. (2001). NonBornOppenheimer calculations on the LiH molecule with explicitly correlated Gaussian functions. Journal of Chemical Physics, 114(8), 33933397.More infoAbstract: The correlated Gaussian basis function is augmented by the addition of premultiplying factors that are powers of the squared internuclear distance. Results of calculations on the LiH molecule are discussed.
 Slanina, Z., Iida, S., Zhao, X., Chiang, L. Y., Adamowicz, L., & Osawa, E. (2001). Computations of hexanitrated and hexaanilinated fullerenes. Synthetic Metals, 121(13), 11011102.More infoAbstract: Oligonitrated and oligoanilinated C60 derivatives offer various application options and thus, computations of their molecular structures, spectra, and stabilities are needed. The paper reports systematic quantumchemical semiempirical (PM3, SAM1) computations of the structures, energetics, and vibrations of the hexaadducts, and relates them to the available experimental data.
 Slanina, Z., Zhao, X., Kurita, N., Gotoh, H., Uhlík, F., Rudziński, J. M., Lee, K. H., & Adamowicz, L. (2001). Computing the relative gasphase populations of C_{60} and C_{70}: Beyond the traditional ΔH_{f,298}^{o} scale. Journal of Molecular Graphics and Modelling, 19(2), 216221.More infoPMID: 11391872;Abstract: Computations and experiments have shown that the relative heat of formation (i.e., the heat of formation per carbon atom) of C70 is lower than of C60. Moreover, various computations suggest that this is actually a general trend among fullerene cages. The relationship is particularly important for gasphase fullerenes. Experiments have shown that C60 is typically more populated than C70 when produced in hightemperature gasphase synthesis. It is not immediately obvious how to reconcile those two terms, or whether the relative heats of formation and the relative populations are in conflict or in agreement. This article deals with this problem, treating it as a general task of relative stabilities of gasphase clusters of different dimensions (i.e., nonisomeric clusters) under different types of thermodynamic equilibria. The results are then applied to C60 and C70 and point out that the conventional standard pressure of 1 atm is considerably different from actual fullerenesynthesis conditions. Apparently, we should expect considerably lower cluster pressures in carbonarc synthesis. At 1 atm, C70 is more populated than C60, but at the conditions of a saturated carbon vapor the stability order is reversed in favor of C60 so that an agreement with experiment is obtained already within the thermodynamic treatment. The pressure effects are modeled using the MNDO, AM1, PM3, and SAM1 quantumchemical semiempirical methods as well as the available experimental data. The computations consistently show that, if the pressure effects are considered, C60 becomes more populated than C70. Relationships of the thermodynamic treatment to more sophisticated but impractical kinetic analysis are also discussed. © 2001 by Elsevier Science Inc.
 Smets, J., Jalbout, A. F., & Adamowicz, L. (2001). Anions of the hydrogenbonded guaninecytosine dimer  Theoretical study. Chemical Physics Letters, 342(34), 342346.More infoAbstract: The quantum mechanical calculations performed in this work revealed an interesting nature of the stationary states of an excess electron in the guaninecytosine (GC) anion. The electron can form either a dipolebound (DB) state or a covalent state. The coexistence of the two states may enhance the ability of the GC dimer to capture an excess electron. © 2001 Elsevier Science B.V.
 Smith, D. M., & Adamowicz, L. (2001). A dynamic model for electron transport in DNA. Journal of Physical Chemistry B, 105(38), 93459354.More infoAbstract: A new model for the simulation of electron transport through duplex DNA is developed and applied to DNA segments of varying lengths consisting of a thymine and complementary adenine strand. The model for electron transport involves moving an excess electron from the 5′ to the 3′ end of the thymine strand by means of the fluctuating geometry of the thymines, using the molecular dynamics method coupled with N1methylthymine vertical electron affinities calculated at the B3LYP/631+G* level of theory. The electron transport lifetimes calculated using this model follow the distance dependence of Jortner's incoherent multistep hopping theory (Jortner, J.; Bixon, M.; Langenbacher, T. Proc. Natl. Aad. Sci U.S.A. 1998, 95, 12759) and were found to decrease with temperature.
 Stepanenko, T., Lapinski, L., Nowak, M. J., & Adamowicz, L. (2001). Infrared spectra of syn and anti isomers of benzaldoxime and pyridine4aldoxime: An experimental matrix isolation and theoretical density functional theory study. Vibrational Spectroscopy, 26(1), 6582.More infoAbstract: Infrared spectra of syn and anti isomers of benzaldoxime and pyridine4aldoxime isolated in lowtemperature Ar and N2 matrixes are reported. The syn isomeric forms of these compounds are highly predominant in the matrixes after deposition. These forms were identified on the basis of their infrared spectra. Upon UV irradiation of the matrixes, new spectra appeared and were assigned to the anti forms of the compounds. The observed photoreactions led to photostationary states. The infrared spectra of the syn and anti isomers were separated and interpreted by comparison with the spectra theoretically calculated at the DFT(B3LYP)/631++G** level. Good agreement between the experimental observations and the theoretical calculations allowed for reliable assignment of the infrared bands in the spectra of the syn and anti isomers of both compounds. © 2001 Elsevier Science B.V. All rights reserved.
 Stepanian, S. G., Reva, I. D., Radchenko, E. D., & Adamowicz, L. (2001). Conformers of nonionized proline. matrixisolation infrared and posthartree  fock ab initio study. Journal of Physical Chemistry A, 105(47), 1066410672.More infoAbstract: Matrixisolation IR spectroscopy and ab initio calculations performed at the DFT, MP2, MP4, and CCSD(T) levels of theory were employed to investigate the conformational topology of the nonionized amino acid proline and its deuterated derivative, N,Odideuteroproline (prolined2). In the calculations, equilibrium structures of 15 lowenergy proline conformers were obtained using the DFT/B3LYP/augccpVDZ and MP2/augccpVDZ methods. The harmonic frequencies and IR intensities of the conformers were calculated for the DFT geometries, and these data was used to account for the zeropoint vibration energy correction and to assist the analysis of the experimental matrixisolation IR spectra. Two proline conformers were found to be present in the Ar matrix. They are the lowest energy conformer with a N⋯H  O H bond (conformer IIa) and the second conformer with a NH⋯O=C H bond (conformer Ia). We found that the DFT/B3LYP and MP2 methods are not capable of predicting the relative energies of the proline conformers with a quantitative accuracy. Both methods provide the energy difference between the IIa and Ia conformers of 78 kJ mol1, thus suggesting that only conformer IIa should be present in the matrix. However, strong bands due to the two conformers are observed in the experimental spectra and their intensities indicate approximately equal presence of the two systems in the matrix. To explain the descrepancy between the MP2 and DFT results and the experiment, calculations were performed at the CCSD(T) level of theory. The relative energy difference obtained at this level of 3.9 kJ mol1 better agrees with the experiment because it is less than kT at the matrix preparation temperature. The observed lowfrequency shift of the OH stretching vibration due to the intramolecular N⋯ H  O H bond in the proline conformer IIa of 534 cm1 is much larger than the ones found for other amino acids (340360 cm1). It demonstrates that the intramolecular H bonding in proline is much stronger then in other amino acids. © 2001 American Chemical Society.
 Adamowicz, L., Malrieu, J., & Ivanov, V. V. (2000). New approach to the statespecific multireference coupledcluster formalism. Journal of Chemical Physics, 112(23), 1007510084.More infoAbstract: Possible forms of the coupledcluster wave function for systems which cannot be described with a singlereference approach are explored. Within the framework of the statespecific multireference coupledcluster theory and based on the complete active space selfconsistent field (CASSCF) wave functions, several proposals are made regarding the form of the reference function and the cluster structure of the CC correlation operator. An internally contracted method provides a viable approach to describing electronic excited states.
 AlJihad, I., Smets, J., & Adamowicz, L. (2000). Covalent anion of the canonical adenine  Thymine base pair. Ab initio study. Journal of Physical Chemistry A, 104(13), 29942998.More infoAbstract: Theoretical ab initio calculations have been performed to determine the stability of the covalent anions of adenine and the adeninethymine (AT) WatsonCrick base pair. The conclusions of this work are as follows: (i) the covalent anion of adenine is predicted to be a stable system with respect to a vertical electron detachment, but unstable with respect to adiabatic detachment; (ii) the covalent anion of the adeninethymine dimer (AT) has similar properties, although in this system the presence of the second base provides an additional stabilization to the excess electron; (iii) in AT the excess electron is localized at the thymine molecule, and this molecule's ring is puckered; (iv) no valence AT anion was found in the calculations with the excess electron located at the adenine molecule; and (v) in view of the above results, we. predict that the AT base pair is not an effective trap of excess electrons. © 2000 American Chemical Society.
 AlJihad, I., Smets, J., & Adamowicz, L. (2000). Isomerism of the covalent anion of the dimer of uracil and 1methylcytosine: Ab initio study. Chemical Physics, 257(23), 167174.More infoAbstract: Theoretical ab initio calculations have been performed to determine the distribution of an excess electron in the covalent anion of the dimer of uracil and 1methylcytosine (UMC). Due to the multitude of possible isomers, this system may present an interesting model for future experimental investigations of the electron attachment and transfer in dimers of nucleic acid bases. The major conclusions of this work are (i) three isomeric structures of the covalent UMC anion have been found in the calculations. The anion where the excess electron is located at the Mc molecule was found to be the most stable; (ii) a noticeable ring distortion was found in the molecule where the excess electron is localized; (iii) the covalent Umc anions are nonplanar, hydrogenbonded systems; and (iv) the outofplane deformed site of the base molecule where the excess electron localizes in the dimer anion is positioned opposite to the site involved in the hydrogen bond. (C) 2000 Elsevier Science B.V.
 Carles, S., Desfrançois, C., Schermann, J. P., Smith, D. M., & Adamowicz, L. (2000). Structures and electron affinities of indole(water)_{N} clusters. Journal of Chemical Physics, 112(8), 37263734.More infoAbstract: Rydberg electron transfer spectroscopy (RET) has been used to determine the dipolebound electron affinity of the indole molecule, and the value of 3 meV was obtained. RET has also been employed to study [indole(water)N] cluster anions and the results have been interpreted with the help of ab initio calculations. It has been shown that for N = 1 and 2 only dipolebound anions are formed and that the electron attachment induces large amplitude motions in these systems. [Indole(water)N] anions with N = 3 and 4 have not been observed. This finding for N = 3 is consistent with a low theoretically predicted dipole moment of the neutral indole(water)3 complex, which is insufficient for the formation of a stable dipolebound anion. Above N = 5, RET experiments showed formation of valence [indole(water)N] anions. From the observed size threshold for the formation of these anions, the negative value of the valence electron affinity of indole equal to  1.03±0.05 meV was deduced. © 2000 American Institute of Physics.
 Dkhissi, A., Adamowicz, L., & Maes, G. (2000). Density Functional Theory Study of the HydrogenBonded Pyridine  H_{2}O Complex: A Comparison with RHF and MP2 Methods and with Experimental Data. Journal of Physical Chemistry A, 104(10), 21122119.More infoAbstract: As a test for the applicability of the density functional theory (DFT) to hydrogenbonded systems, various local and nonlocal exchange correlation functionals have been used to calculate the equilibrium structure and the harmonic vibrational frequencies of the pyridine  water complex. We also performed restricted Hartree Fock (RHF) and Moller  Plesset (MP2) calculations for comparison. Three different basis sets (631G**, 631++G**, and 6311++G(2d, 2p) 6d)) were used in the study. The best agreement between the DFT computed and the experimental rotational constants for monomeric pyridine were obtained with the B3PW91 and B3LYP functionals. For the vibrational frequencies of the Hbonded complex, the experimental data from matrix FTIR spectrometry as well as the results obtained from the ab initio methods were best reproduced with the DFT method with the nonlocal exchange and correlation functionals. The hybrid approaches, which mix HF and Slater exchange, are particularly effective in these types of calculations.
 Dkhissi, A., Adamowicz, L., & Maes, G. (2000). Hybrid density functionals and ab initio studies of 2pyridoneH_{2}O and 2pyridone(H_{2}O)_{2}. Chemical Physics Letters, 324(13), 127136.More infoAbstract: Theoretical calculations have been carried out on the interaction between water and the carbonyl base 2pyridone. The calculations have been performed with two hybrid density functionals (B3PW91 and B3LYP). The comparison with experimental gas phase data as well as with results from the ab initio methods RHF and MP2 shows good performance of these functionals, especially for B3LYP. Two stable structures were found for the 2pyridoneH2O complex: the first one is a closed complex with two hydrogen bonds, C=OHOHN, while the second structure is an open complex, C=OHOH. The theoretical results indicate that the closed complex is the most stable one due to a cooperative effect between the two Hbonds. The structure of the 2pyridone(H2O)2 heterotrimer is also closed with three hydrogen bonds, C=OHOHOHN.
 Dkhissi, A., Adamowicz, L., & Maes, G. (2000). Theoretical investigation of the interaction between 2pyridone/2hydroxypyridine and ammonia. Journal of Physical Chemistry A, 104(23), 56255630.More infoAbstract: The structures and harmonic vibrational frequencies of the closed complexes 2pyridoneNH3 (2PYNH3), 2hydroxypyridineNH3 (2HPNH3), 2pyridone(NH3)2 (2PY(NH3)2), and 2hydroxypyridine(NH3)2 (2HP(NH3)2) were studied using the density functional theory (DFT). The final energies of each complex were obtained using the B3LYP and MP2 methods. The NH3 molecule is both a proton donor and a proton acceptor in the 2PYNH3 and 2HPNH3 complexes, while in the 2PY(NH3)2 and 2HP(NH3)2 systems, the ammonia dimer acts both as a proton donor and a proton acceptor. Comparison is made between the theoretical results and recent experimental results for the 2PYNH3 and 2PY(NH3)2 complexes. In the heterotrimer complexes, the cooperative effects are substantial and this is reflected by the intermolecular distance of the ammonia dimer, by the elongation of the NH bond length, by the frequency shift of the v(NH) mode, and by the additive interaction energy. © 2000 American Chemical Society.
 Dkhissi, A., Houben, L., Smets, J., Adamowicz, L., & Macs, G. (2000). Theoretical calculations and matrixisolation FTIR studies of hydrogenbonded complexes of molecules modeling cytosine or isocytosine tautomers. 7. 2Hydroxypyridine/2oxopyridine complexes with H_{2}O. Journal of Physical Chemistry A, 104(43), 97859792.More infoAbstract: The Hbond interaction between the tautomeric cytosinemodeling system, 2hydroxypyridine (2HP)/2oxopyridine (2OP), with water is investigated using matrixisolation FTIR spectroscopy and ab initio calculations performed with the RHF and MP2 methods and the DFT method with the B3LYP hybrid functional. Equilibrium structures of six different complexes have been found in the calculations. The theoretical results indicate that the closed N⋯HO⋯HO and C=O⋯HO⋯HN Hbonded water complexes are the most stable systems for the 2HP and the 2OP tautomers, respectively. Both closed complexes as well as the three open complexes OH⋯OH2, N⋯HOH, and C=O⋯HOH have been identified in the experimental spectrum. A detailed analysis of the structural, energetic and vibrational parameters for the closed complexes has been performed. Both in the matrix (Ar) and in the gas phase, the hydroxy tautomer, 2HP, is more stable than the oxo form, 2OP. An important observation is that addition of a single water molecule shifts the tautomeric equilibrium toward the oxo 2OP form, which is in agreement with the theoretical calculations. © 2000 American Chemical Society.
 Dkhissi, A., Ramaekers, R., Houben, L., Adamowicz, L., & Maes, G. (2000). DFT/B3LYP study of the hydrogenbonding cooperativity: Application to (2pyridone) _{2} , 2pyridone H_{2}O , 2pyridone CH_{3}OH and 2pyridone CH_{3}OCH_{3}. Chemical Physics Letters, 331(56), 553560.More infoAbstract: DFT/B3LYP(631++G**) calculations are used to provide information on closed NHOHbonded complexes, containing two hydrogen bonds. The studied model systems with two Hbonds are the 2pyridone dimer, 2pyridoneH2O and 2pyridoneCH3OH. For the sake of comparison, the open, singly Hbonded complex 2pyridoneCH3OCH3 is also examined. In the closed Hbonded complexes, an Hbond cooperativity effect has been observed, but it depends on the linearity of the hydrogen bond NHO. For the 2pyridone dimer, which has the perfect, linear geometry, the strongest cooperativity is observed, whereas in all the other closed Hbonded systems, the cooperativity is reduced by deformation from the linear Hbond geometry. These observations are illustrated by different correlations established between the Hbond angle and other characteristic Hbond parameters. © 2000 Elsevier Science B.V.
 Forés, M., Duran, M., Solà, M., & Adamowicz, L. (2000). Theoretical Characterization of Intramolecular Proton Transfer in the Ground and the LowestLying Triplet Excited States of 1Amino3Propenal: A Methodological Comparison. Journal of Computational Chemistry, 21(4), 257269.
 Galetich, I., Stepanian, S. G., Shelkovsky, V., Kosevich, M., Blagoi, Y., & Adamowicz, L. (2000). Combined mass spectrometric and Ab initio study of the point contacts between 9methyladenine and the amide group. Journal of Physical Chemistry A, 104(39), 89658971.More infoAbstract: We applied the temperaturedependent field ionization mass spectrometry method to determine the interaction energy between 9methyladenine and acrylamide. Acrylamide mimics the side chain amide group of the natural amino acids asparagine and glutamine. The experimental enthalpy of the dimer formation derived from van't Hoff's plot is 52.0 ± 5.0 kJ mol1. This value is close to the interaction energy between acrylamide and 1methylcytosine (57.0 kJ mol1) and much higher than the interaction energy between acrylamide and 1methyluracil (40.6 kJ mol1). The DFT/B3LYP/631++G** and MP2/631++G** quantumchemical methods were used to determine the structures and the interaction energies of the 9methyladenineacrylamide dimers. In the calculations, two energetically almost equivalent equilibrium structures were found. They are stabilized by intermolecular Hbonds formed between the acrylamide amide group and the WatsonCrick and Hoogsteen sites of 9methyladenine, respectively. The calculated interaction energies of the dimers are 41.4 and 39.2 kJ mol1, respectively, at the DFT level of theory and 40.9 and 38.8 kJ mol1, respectively, at the MP2 level. We also performed calculations for 9methyladenineglutamine dimers at the DFT/B3LYP/631++G** level of theory. The interaction energy differences between the corresponding 9methyladenineacrylamide and 9methyladenineglutamine dimers of less than 1 kJ mol1 were obtained in the calculations. This result demonstrates that acrylamide is a good model for the amino acid amide group. The experimantal and the calculated data indicate that the amino acid amide group can interact with the adenine residue in the singlestranded DNA, as well as in the major groove of doublestranded DNA. The influence of the interaction with the amide group on the stability of the adenineuracil base pair is discussed. © 2000 American Chemical Society.
 Ivanov, V. V., & Adamowicz, L. (2000). CASCCD: coupledcluster method with double excitations and the CAS reference. Journal of Chemical Physics, 112(21), 92589268.More infoAbstract: A new statespecific coupledcluster method, the CASCCD, was developed. Algebraic expressions for the energy and amplitude equations were derived and represented with a diagrammatic technique. By performing numerical tests, it was proven that the method is capable of correctly describing the stretching process in systems which separate into openshell fragments.
 Kinghorn, D. B., & Adamowicz, L. (2000). High accuracy nonBornOppenheimer calculations for the isotopomers of the hydrogen molecule with explicitly correlated Gaussian functions. Journal of Chemical Physics, 113(10), 42034205.More infoAbstract: The nonadiabatic ground states of six isotopomers of hydrogen were examined using the nonBornOppenheimer method with the explicitly correlated Gaussian functions. The energy results for the heteronuclear systems were found to be more accurate than that for the homonuclear systems.
 Lvanov, V. V., & Adamowicz, L. (2000). New scheme for solving the amplitude equations in the statespecific coupled cluster theory with complete active space reference for ground and excited states. Journal of Chemical Physics, 113(19), 85028513.More infoAbstract: A new scheme for solving the coupled cluster (CC) amplitude equations for ground and excited electronic states in the stateselective multireference (MR) CC method based on the complete active space (CAS) reference wave function (CASCC) is proposed. The CASCC wave function is generated using a single formal reference determinant, which is one of the CAS determinants, as the origin of the configuration expansion. Some single and double excitations from other CAS determinants may be triple, quadruple, and higher excitations from the reference determinant. We show that one may include the contribution from these higher excitations indirectly by modifying the coupled cluster amplitude equations corresponding to the single and double excitations. The modification involves including projections against the higher excitations in the equations for the singles and doubles. Test calculations for the ground and the first excited state of the H8model system and for the singlettriplet splitting of the CH2biradical shows that the results improve with the use of the new scheme and become to the FCI benchmarks. © 2000 American Institute of Physics.
 Slanina, Z., Miyajima, C., Zhao, X., Adamowicz, L., & Osawa, E. (2000). Computations of metalcovered C_{60} and C_{70}. Computational Materials Science, 18(34), 308314.More infoAbstract: Semiempirical quantumchemical PM3 calculations are reported for a new class of exohedral metallofullerenes metalcoated or metalcovered fullerenes: C60Mn and C70Mn. The exohedral species have been observed in gas phase, however, their geometrical and electronic structures are not known yet. Relativelyeven metalatom distributions over the fullerene rings are considered  such regular forms are computed for M = Be, Mg, Al. Three selected types of stoichiometries are treated in particular: C60M12/C70M12 (metal atoms above all pentagons), C60M20/C70M25 (metal atoms above all hexagons), and C60M32/C70M37 (metal atoms above all rings). If an odd number of electrons should result (Al), the related cation is computed, or one metal atom added or removed. This interesting arrangement above the rings is possible only for some types of atoms, while other elements are localized above bonds or atoms, or inside the cage, or even react and destroy the cage. Other limitation comes from the parametrization of the computational technique used  the PM3 semiempirical method is parametrized only for some selected metals. Metallayer atomization heats are suggested as a stability measure. Structural characteristics are also presented and interesting reductions of the cage symmetry are found. Their relationship to JahnTeller effect is discussed. The metal covered fullerenes can represent models for metal catalysis in the nanotube synthesis and could eventually lead to new interesting materials. © 2000 Elsevier Science B.V. All rights reserved.
 Slanina, Z., Miyajima, C., Zhao, X., Uhlík, F., Adamowicz, L., & Osawa, E. (2000). Metalcoated fullerenes C_{60}M_{n}: calculations for M = Be, Mg, Al and n = 12, 20, 32. Fullerene Science and Technology, 8(4), 385402.More infoAbstract: Semiempirical quantumchemical PM3 calculations are reported for a relatively new class of exohedral metallofullerenes  metalcoated or metalcovered fullerenes: C60Mn. The exohedral species were recently observed, however, their geometrical and electronic structure is not known yet. In this paper, relativelyeven metalatom distributions over the fullerene rings are considered  such regular forms are computed for M = Be, Mg, Al. Three selected stoichiometries are treated: C60M12, C60M20, and C60M32. The stoichiometries correspond to the location of the metal atoms above the twelve pentagons, above the twenty hexagons, and above each of the thirty two rings of C60. This interesting arrangement over the rings is possible only for some types of atoms, while other elements are localized above bonds or atoms, or inside the cage, or even react and destroy the cage. Other limitation comes from the parametrization of the computational methods  the computations are performed with the PM3 semiempirical method and metallayer atomization heats are used as a stability measure. Structural characteristics are presented, too. Considerable reductions of the cage symmetry are reported and their relationships to JahnTeller effect are discussed, too (no case of the icosahedral symmetry is found).
 Slanina, Z., Zhao, X., Osawa, E., & Adamowicz, L. (2000). Cyclic and linear structures of C_{13}: a computational study. Fullerene Science and Technology, 8(4), 369383.More infoAbstract: Recently, a linear structure of C13 was observed and its infrared spectrum recorded. Computations of the linear and cyclic structures of C13 are thus presented. The computations are performed at ab initio level using the standard 631G* basis set. Electron correlation is treated by density functional theory (Becke3LYP) and perturbation treatment (MP2=FC). With the densityfunctional approach the standard 6311G* and 6311G(2d,p) basis sets are applied, too. In all the treatments the cyclic structure (C2v symmetry) is located about 90 kJ/mol below the linear form (D∞h symmetry) while a tricyclic structure is considerably higher in energy (about 280 kJ/mol above the monocyclic species). Even at high temperatures the linear form represents less than 10% of the equilibrium isomeric mixture. Computed IR spectra are also reported. The stability evaluation corresponds to the findings of ion chromatography.
 Smith, D. M., Jalbout, A. F., Smets, J., & Adamowicz, L. (2000). Cytosine anions: Ab initio study. Chemical Physics, 260(12), 4551.More infoAbstract: Theoretical ab initio calculations have been performed to determine the stability of covalent and dipolebound anions of cytosine. The work is related to the recent anion spectroscopy experiments performed on some nucleic acid bases including cytosine by Schiedt et al. [Chem. Phys. 239 (1998) 511], where two anion states attributed to dipolebound anions of the aminohydroxy (AH) and aminooxo (AO) tautomers were detected. The present calculations reveal that (i) the AO isomer and two rotamers of the AH isomer have sufficient dipole moments to bind σ excess electrons in stationary dipolebound states. The calculated adiabatic electron affinities are 58, 22, and 6 meV for the AO cytosine and the two rotamers of the AH cytosine, respectively. These values are considerably smaller than the two experimentally determined values of 85 ± 8 and 230 ± 8 meV; (ii) our calculations also describe covalent anions of the AO and AH tautomers. In both systems, the sixmember ring is noticeably distorted from planarity. According to the calculations only, the AO covalent anion is vertically stable with respect to the electron attachment, and the corresponding vertical electron detachment energy is 102 meV. However, both AO and AH cytosine anions are predicted to be unstable with respect to the adiabatic electron detachment. This finding is consistent with the lack of a covalent anion feature in the experimental spectrum. (C) 2000 Elsevier Science B.V.
 Alexandrov, V., Filippov, I., Adamowicz, L., & Salzman, W. R. (1999). Passing dynamic objects by reference in Fortran 90: Application to timedependent quantum chemistry. Computers and Chemistry, 23(1), 2531.More infoAbstract: A new effective strategy of passing large variable size objects to subroutines and back in FORTRAN 90 is described. The objects are passed by reference utilizing userdefined pointer type variables declared in a shared module. The efficiency of the developed approach is demonstrated by applying it to program the analytic solution for action of the powers of Hamiltonian on the Hermite polynomial type basis function. © 1999 Elsevier Science Ltd. All rights reserved.
 Bajdor, K., Glice, M. M., Leś, A., Adamowicz, L., & Kołos, R. (1999). Vibrational spectroscopy of glutethimide  From isolated molecule to solid state. Journal of Molecular Structure, 511512, 107116.More infoAbstract: The IR and Raman spectra of glutethimide (3ethyl3phenyl2,6 piperidinedione) were obtained for both monomers and dimers in solution and matrix environment. The IR and Raman spectra were also obtained for the crystalline glutethimide. The spectral assignments were made based on quantum chemical theoretical calculations with the use of RHF and DFT methods and using isotopically labelled derivatives. The Raman spectra seem to be of limited use to study the intermolecular interactions of cyclic imides. The dominant species in diluted solutions of CCl4, CS2 and CHCl3 solutions were the glutethimide monomers. The dimer formation appeared at higher concentration about 0.2% in CCl4 and CS2 and 1% in CHCl3. In CH3CN solution no dimers were formed. The band corresponding to the v(NH) stretching vibration was selected as a diagnostic band reflecting hydrogen bonding interactions of glutethimide with the solution, matrix and crystalline environment.
 Borowiec, M. T., Szymczak, H., Zaleski, M., Kaczor, P., Adamowicz, L., Strzeszewski, J., Watterich, A., & Kovacs, L. (1999). Raman and IR spectroscopy investigations of double tungstates. Proceedings of SPIE  The International Society for Optical Engineering, 3724, 288291.More infoAbstract: A framework is introduced for identifying the major uptake process and further interpreting measurements of plant concentrations. The mathematical analysis leads to two interpretive tools, each containing three distinct and easily recognizable segments, and each segment corresponds to one of the three deposition processes. When the experimental data are plotted and the three segments are identified, it is possible to determine the dominant uptake process for a given compound, thus making it possible to further interpret plant uptake behavior.
 Desfrançois, C., AbdoulCarime, H., Carles, S., Périquet, V., Schermann, J. P., M., D., & Adamowicz, L. (1999). Experimental and theoretical ab initio study of the influence of Nmethylation on the dipolebound electron affinities of thymine and uracil. Journal of Chemical Physics, 110(24), 1187611883.More infoAbstract: The influence of Nmethylation on the dipolebound electron affinities of pyrimidine nucleic acid bases, uracil and thymine, has been investigated theoretically using ab initio quantum mechanical calculations, and experimentally using Rydberg electron transfer spectroscopy. Both experiment and theory are consistent in showing that replacement of hydrogen atoms by methyl groups reduces electron affinities corresponding to formation of dipolebound anions of these systems. Also, the distortion of the anion geometries with respect to the geometries of the neutral parents are reduced with the methylation. © 1999 American Institute of Physics.
 Desfrançois, C., Périquet, V., Carles, S., Schermann, J. P., Smith, D. M., & Adamowicz, L. (1999). Experimental and ab initio theoretical studies of electron binding to formamide, Nmethylformamide, and N,Ndimethylformamide. Journal of Chemical Physics, 110(9), 43094314.More infoAbstract: The influence of methylation upon adiabatic electron affinities of formamide (F), Nmethylformamide (NMF), and N,Ndimethylformamide (DMF) is experimentally investigated by means of Rydberg electron transfer spectroscopy and calculated with the use of highlevel ab initio methods. In the anions of these systems the excess electrons are captured in diffuse dipolebound states. The methylation of formamide results in a slight increase of the dipole moment and in an increased molecular size. The two factors have opposite effects on the electron affinity. Both experimental data and theoretical results are in agreement, showing that the molecular size effect dominates and that the electron affinity noticeably decreases with the methylation. © 1999 American Institute of Physics.
 Dkhissi, A., Houben, L., Ramaekers, R., Adamowicz, L., & Maes, G. (1999). Matrixisolation FTIR studies and theoretical calculations of different types of hydrogenbonding: 2hydroxypyridine/2oxopyridine complexed with HCl. Journal of Physical Chemistry A, 103(50), 1102011025.More infoAbstract: The Hbond interaction of the cytosine model compound 2hydroxypyridine and its tautomer 2oxopyridine with HC1 is investigated using the combined matrixisolation FTIR and theoretical density functional and ab initio methods. The theoretical calculations have been carried out at the B3LYP/631++G** and RHF/631++G** levels of theory. Different types of hydrogenbonding have been found: two closed complexes of the proton transfer type, each containing two hydrogen bonds, i.e., N+H⋯Cl⋯HO and C=O+ H⋯Cl⋯HN; two open complexes of intermediate strength, N⋯H Cl and C=O⋯H Cl; and one weak complex, HO⋯HCl. The theoretical results indicate that the closed Hbonded complexes are the most stable systems for both the hydroxy and the oxo tautomers. The increased stability of these complexes is due to a cooperative Hbonding effect. The experimental spectra are consistent with this prediction, but the weaker complexes are also identified. A comparison of the experimental and calculated IR frequencies demonstrates that the frequency shifts of the vibrational modes directly involved in the Hbond interactions, especially the XH stretching modes, are better predicted by the DFT method than by the RHF method. For the other vibrational modes not directly involved in the Hbonds, the RHF methodology has a similar accuracy compared to the DFT method. © 1999 American Chemical Society.
 Dkhissi, A., Houben, L., Ramaekers, R., Adamowicz, L., & Maes, G. (1999). Theoretical density functional and ab initio computational study of different types of hydrogenbonding. Journal of Molecular Structure: THEOCHEM, 487(12), 2731.More infoAbstract: Computational calculations have been carried out at the B3LYP/631 + + G** and RHF/631 + + G** levels of theory to determine the equilibrium structures of complexes formed between the tautomers 2hydroxypyridine and 2 oxopyridine with HCl. Different types of hydrogenbonding have been found: two closed complexes of type III (proton transfer), each containing two hydrogen bonds i.e. NH+···Cl···HO and C=OH+···Cl···HN, two open complexes of intermediate type I → II: N···H···Cl and C=O···HCl, and one open complex of type I:HO···HCl. A comparison of the experimental and calculated IR frequencies demonstrates that the most accurate spectral parameters are predicted by the B3LYP/631 + + G** method, especially when this method is used with a set of different scaling factors.
 Dkhissi, A., Houben, L., Smets, J., Adamowicz, L., & Maes, G. (1999). Density functional theory and abinitio computational study of the 2 hydroxypyridine/2pyridone system: A comparison with FTIR data from matrix isolation experiments. Journal of Molecular Structure, 484(13), 215227.More infoAbstract: The tautomeric equilibrium 2hydroxypyridine mutually implies 2pyridone was observed in a lowtemperature Ar matrix. The infrared spectra of the tautomers were analysed and assigned using DFT/(B3LYP and B3PW91)/631 ++G** and RHF/631++G** methods. The molecular parameters (rotational constants and dipole moments) are calculated with different methods and compared with recent gasphase microwave data. The rotational constants and dipole moments predicted using the DFT method agree much better with the experimental results than the parameters predicted at the HF level. The energy difference for the tautomeric equilibrium and the tautomerization constant were calculated and these parameters are compared with recent theoretical and experimental predictions.
 Forés, M., & Adamowicz, L. (1999). A CASSCFCASPT2 study of the excitedstate intramolecular proton transfer reaction in 1amino3propenal using different active spaces. Journal of Computational Chemistry, 20(13), 14221431.More infoAbstract: In this work we analyze how the choice of the active space in the CASSCF (the completeactivespace multiconfiguration selfconsistentfield method) and CASPT2 (the secondorder perturbation theory based on the CASSCF reference wave function) calculations affects the computed potential energy curves (PECs) for the intramolecular proton transfer reaction in the ground state and the two lowest lying singlet excited states of 1amino3propenal. As anticipated, the results revealed that, qualitatively, the proton transfer in the different states can be correctly described even by minimal active spaces, which include the orbitals involved in the electronic excitation of the considered state and the antibonding sigma orbital corresponding to the bond formed by the molecule with the migrating hydrogen atom. However, quantitatively, the relative energies of the two tautomers and the energy barriers computed at the CASSCF level change when the active space is increased, indicating importance of the dynamic electron correlation. Introducing the dynamic correlation effects via CASPT2 makes the calculated energy parameters more uniform among the different active spaces. The analysis suggested certain optimal active spaces for studying proton transfer reactions in systems similar to 1amino3propenal. The PEC calculations for excited states showed that the results are sensitive to the molecular geometries used in the calculations, particularly near the transition point. © 1999 John Wiley & Sons, Inc.
 Galetich, I., Kosevich, M., Shelkovsky, V., Stepanian, S. G., Blagoi, Y., & Adamowicz, L. (1999). Structure and energy of nucleic acid baseamino acid complexes: 1. 1 methyluracilacrylamide. Journal of Molecular Structure, 478(13), 155162.More infoAbstract: The temperaturedependent field ionization mass spectrometry method was applied to determine the interaction energy between 1methyluracil and acrylamide, which mimics the side chains of the natural aminoacids Asparagine and Glutamine. The experimental enthalpy of formation of 1methyluracil acrylamide dimer derived from VantHoff plots is  40.6 ± 4.2 kJ mol1. Two equilibrium geometry configurations of the 1methyluracilacrylamide dimer stabilized by intermolecular NH···O Hbonds were found via quantum chemical calculations at the DFT/B3LYP/631 + +G** and MP2/631 +G* levels of theory. The two methods yielded similar interaction energies for the two configurations, which are in good agreement with the experimental result.
 Galetich, I., Stepanian, S. G., Shelkovsky, V., Kosevich, M., Blagoi, Y., & Adamowicz, L. (1999). Structures and interaction energies of nucleic acid baseamino acid complexes. Methylcytosinesacrylamide model. Journal of Physical Chemistry B, 103(50), 1121111217.More infoAbstract: The interaction energies between the cytosine methylated derivatives (m1Cyt, m21,NCyt, m31,N,NCyt) and acrylamide, which models the side chains of natural amino acids asparagine and glutamine, were measured using the temperaturedependent field ionization mass spectrometry method. The experimental enthalpies of the dimer formation are 57.0 kJ mol1 for the acrylamidem1Cyt dimer, 58.7 kJ mol1 for the acrylamidem21,NCyt dimer and 45.7 kJ mol1 for the acrylamidem31,N,NCyt dimer. The dimer structures were determined using quantumchemical calculations. The calculated structures are almost planar with antiparallel orientations of the monomer dipoles and they are stabilized by intermolecular NH⋯O and NH⋯N Hbonds. The calculated interaction energies are in good agreement with the experimental data. The results indicate that the interaction energy between acrylamide and m1Cyt is significantly higher than that between acrylamide and m1Uracil. This result is relevant to the mechanism of the proteinnucleic acid recognition. © 1999 American Chemical Society.
 Kinghorn, D. B., & Adamowicz, L. (1999). A correlated basis set for nonadiabatic energy calculations on diatomic molecules. Journal of Chemical Physics, 110(15), 71667175.More infoAbstract: A new explicitly correlated basis set suitable for nonadiabatic energy calculation on small diatomic molecules is presented. The basis functions consist of correlated Gaussians multiplied by powers of the internuclear distance, Nbody formulas for Hamiltonian matrix elements and energy gradient components are derived and presented along with a discussion of the nonadiabatic Hamiltonian and symmetry considerations. A sample calculation is presented for the ground state energy of the benchmark system H^ in which rapid convergence to near exact results was observed. © 1999 American Institute of Physics.
 Kinghorn, D. B., & Adamowicz, L. (1999). Beyond the bornoppenheimer approximation. Theoretical and Computational Chemistry, 6(C), 2146.More infoAbstract: In this review we discuss how one can approach atomic and molecular quantummechanical calculations without assuming the clampnuclei approximation. Although such calculations are very rare for chemically interesting systems, the progress in conceptual formulation of the theory in this area and, more importantly, in development of necessary computational tool has progressed to the point where nonBornOppenheimer it will completely undermine, traditional approaches to the problems of molecular spectroscopy which are based on the concept of the Potential Energy Surface (PES). It may also provide new insight in such important concepts of chemistry as Chemical Bonding and Molecular Structure, which have been the central points in the works of L. Pauling [1]. © 1999 Elsevier B.V. All rights reserved.
 Kinghorn, D. B., & Adamowicz, L. (1999). Improved nonadiabatic groundstate energy upper bound for dihydrogen. Physical Review Letters, 83(13), 25412543.More infoAbstract: A new, highly accurate, value for the nonadiabatic energy of the dihydrogen ground state is reported, (1.1640250232 hartree). The calculations were performed with a direct nonadiabatic variational approach using a new correlated Gaussian basis set including powers of the internuclear distance. © 1999 The American Physical Society.
 M., D., Smets, J., & Adamowicz, L. (1999). Ab initio theoretical study of dipolebound anions of molecular complexes: Water pentamer anions. Journal of Chemical Physics, 110(8), 38043810.More infoAbstract: Results of ab initio calculations of the (H2O)5/(H2O)5 systems are presented. The main conclusions of the study are as follows: Based on the calculations we predict that water pentamer anions are metastable systems in agreement with their weak spectral manifestation in the gasphase experiments of Bowen and coworkers; the excess electrons in all five structural isomers of the water pentamer anion found in the calculations are attached to the clusters by virtue of a dipoleelectron interaction; the (H2O)5 anions found in the calculations are close in energy but have different vertical electron detachment energies ranging from 143 to 395 meV. © 1999 American Institute of Physics.
 M., D., Smets, J., & Adamowicz, L. (1999). Anions of the HydrogenBonded Uracil Dimer. Ab Initio Theoretical Study. Journal of Physical Chemistry A, 103(29), 57845790.More infoAbstract: Theoretical ab initio calculations have been performed to determine the ability of the hydrogenbonded uracil dimer to form stable anions. The major conclusions of this work are (i) three of the hydrogenbonded conformers of the uracil dimer can form stable dipolebound anions with excess electrons; (ii) uracil dimer can form a covalent anion that has a structure dissimilar from the structures of the neutral dimer; (iii) in the covalent uracil dimer anion the excess electron is localized on one of the uracil molecules and this molecule shows an outofplane distortion; (iv) the covalent uracil dimer anion is stable with respect to a vertical electron detachment, but at the level of theory (MP2) used in the calculations the anion is marginally unstable with respect to an adiabatic electron detachment.
 M., D., Smets, J., & Adamowicz, L. (1999). Isomerism of the Covalent Anion of the UracilThymine Dimer. Ab Initio Study. Journal of Physical Chemistry A, 103(21), 43094312.More infoAbstract: Theoretical ab initio calculations have been performed to determine the ability of the uracilthymine (UT) dimer to form stable covalent anions. The are two major conclusions of this work (i) Two isomeric structures of the covalent UT dimer have been found in the calculations. The anion, where the excess electron is located at the uracil molecule, is 1.4 kcal/mol more stable than the form where the excess electron is located at the thymine molecule, (ii) The ring of the molecule where the excess electron is localized in noticeably nonplanar.
 Plokhotnichenko, A. M., Radchenko, E. D., Stepanian, S. G., & Adamowicz, L. (1999). Pquinone dimers: Hbonding vs stacked interaction. Matrixisolation infrared and ab initio study. Journal of Physical Chemistry A, 103(50), 1105211059.More infoAbstract: Matrixisolation IR spectroscopy and ab initio calculations have been used to investigate the structure and the vibrational spectrum of the quinone dimer formed in lowtemperature Ar matrices. A specially developed experimental technique was applied to separate the bands of the quinone dimer from the bands of the quinone monomer in the IR spectra. The compositionof the matrix samples was precisely controlled with a lowtemperature quartz micr.obalance. As a result, a set of bands assigned to the quinone dimer were identified. Ab initio calculations at the MP2/631+G*, MP2/631++G**, and DFT/B3LYP/631++G** levels of theory have been carried out to determine the relative energies and the vibrational spectra of the two stable configurations of the quinone dimer found in the calculations. These configurations are a planar complex with two weak CH⋯O hydrogen bonds and a stacked complex stabilized by the dispersion forces. The MP2 calculations of the interaction energies corrected for the basis set superposition error (BSSE) predict the two dimers to be equally stable. The comparison of the observed IR frequency shifts with the theoretically predicted shifts indicate that only the planar configuration is responsible for all of the experimentally observed dimer bands. Thus, we conclude that the stacked dimer is absent from the matrix. The influence of the matrix environment on the stability shift in favor of the planar dimer is discussed. © 1999 American Chemical Society.
 Ramaekers, R., Smith, D. M., Smets, J., & Adamowicz, L. (1999). Ab initio theoretical study of dipolebound anions of molecular complexes (HF)(n)/^{}, n = 59. Journal of Molecular Structure, 513(13), 169176.More infoAbstract: Ab initio calculations have been performed to determine structures and electron detachment energies of the hydrogen fluoride hydrogenbonded polymers, (HF)(n)/, n = 59. In these systems, the excess electron is bound by virtue of the dipoleelectron interaction. It was found that, unlike the neutral complexes which form cyclic structures, the equilibrium geometries of the anions have 'zigzag' linear shapes. For all the five complexes, the predicted vertical detachment energies are positive (307, 363, 405, 437, and 461 meV for (HF)5/, (HF)6/, (HF)7/, (HF)8/, and (HF)9/, respectively). These results were obtained at the MollerPlesset secondorder level of theory at the geometries of the anions calculated at the Hartree Fock level. The calculations also indicate that up to n = 8, the (HF)(n)/ anions are metastable systems with respect to electron detachment and simultaneous transformation of the cluster to the most stable cyclic configurations. However, calculations predict that starting from (HF)9/ the linear HF cluster anions are adiabatically stable systems.
 Schoone, K., Maes, G., & Adamowicz, L. (1999). Computational and matrixisolation FTIR study of the tautomeric and vibrational properties of methylated guanines. Journal of Molecular Structure, 480481, 505509.More infoAbstract: The tautomeric and vibrational characteristics of 1,7dimethylguanine (17DMG), 7methylguanine (7MG) and 9methylguanine (9MG) are studied by a combined experimental matrixisolation FTIR and a computational study. The theoretical methods used are the RHF, MP2 and DFT/B3LYP method with the 631 +(+) G** basis set. For 17DMG, only the most stable aminooxo tautomer form can be detected in the FTIR spectrum. The FTIR spectra of 7MG and 9MG reveal the presence of two tautomers in the matrix in detectable amounts: the aminooxoN1H form and the aminohydroxy form. The amount of the amino hydroxy form of 7MG is very small (≃ 1%), while the aminohydroxy form of 9MG, for which the two rotamers can be distinguished in the spectrum, is as abundant as the aminooxo form. A comparison of the experimental and calculated IR frequencies demonstrates that the most accurate spectral parameters are predicted by the DFT/B3LYP method, especially when this method is used with a set of different scaling factors.
 Smets, J., McCarthy, W., Maes, G., & Adamowicz, L. (1999). Correlations between ab initio and experimental data for isolated 1:1 hydrogenbonded complexes of pyridine and imidazole derivatives with water. Journal of Molecular Structure, 476(13), 2743.More infoAbstract: Correlations between selected ab initio predicted and experimentally observed properties of 1:1 Hbonded complexes of a series of pyridine derivatives with water are investigated. Specifically, relationships are found between the experimentally observed properties of proton affinity, frequency shift, and dipole moments, and the ab initio calculated values of bond distance, interaction energy, frequency shift, IR absorption intensity and charge differences obtained from Mulliken population analysis. The examined correlations were then compared with data collected for pyridine H2O and imidazoleH2O complexes. Similarities were found between the correlations in the theoretically determined properties and the correlations in experimentally observed parameters. It was also found that welldefined relations could be established between the calculated and observed properties for the pyridine derivatives, but these could not be reliably extended to the imidazole systems. These similarities demonstrate that the presently available ab initio calculations are useful predictors of experimental behavior of Hbonded systems but only for chemically closely related molecules.
 Smith, D. M., Smets, J., Elkadi, Y., & Adamowicz, L. (1999). Ab initio theoretical study of dipolebound anions of molecular complexes: Formaldehyde dimer anion. Chemical Physics Letters, 305(12), 169177.More infoAbstract: Theoretical ab initio calculations reveal that when two formaldehyde molecules form a cluster with a collinear orientation of their dipoles and a perpendicular orientation of their planes, an excess electron can attach to the system forming a dipolebound anion. This configuration is a local minimum on the anion potential energy surface (PES) and the vertical electron detachment energy of the anion at this point is predicted to be 20 meV. However, calculations of the PES near the minimum revealed that it does not support the zeropoint vibration corresponding to the bending mode of the complex.
 Adamowicz, L., Piecuch, P., & Ghose, K. B. (1998). The stateselective coupled cluster method for quasidegenerate electronic states. Molecular Physics, 94(1), 225234.More infoAbstract: This presentation will provide a comprehensive account of the recent developments in the the stateselective coupled cluster method for quasidegenerate electronic states. We will mention some previous important applications and present new data from our recently finished calculations related to stretching of chemical bonds. We will also discuss some possible future developments.
 Alexandrov, V., Smith, D. M., Rostkowska, H., Nowak, M. J., Adamowicz, L., & McCarthy, W. (1998). Theoretical study of the OH stretching band in 3hydroxy2methyl4pyrone. Journal of Chemical Physics, 108(23), 96859693.More infoAbstract: The infrared spectrum of 3hydroxy2methyl4pyrone reveals an OH stretching frequency roughly 200 cm1 lower than that of a typical alcohol group. The frequency lowering results from intramolecular hydrogen bonding between the alcohol and ketone groups. In this work, the stretching and bending vibrations of the OH group in 3hydroxy2methyl4pyrone are studied with a theoretical methodology more rigorous than the conventional harmonic approximation. A twodimensional potential energy surface in internal coordinates corresponding to different hydrogen positions in the plane of the molecule is calculated with the use of the second order MøllerPlesset perturbation theory. To include all possible variations in kinetic energy in a large amplitude vibrational mode, gmatrix elements with variable values are employed. The analytical expression for the Hamiltonian matrix elements of the twodimensional vibrational problem in a basis of shifted Gaussian functions is derived. Expectation values for the OH stretch nuclear states are variationally determined with the use of shifted Gaussian functions as the basis set. The results of the calculations are compared with the recent matrixisolation infrared (IR) spectroscopic results. The calculated transition frequency corresponding to the inplane OH stretching is found to be in good agreement with the experimental value. © 1998 American Institute of Physics.
 Alexandrov, V., Stepanian, S., & Adamowicz, L. (1998). Theoretical ab initio study of OH vibrational band in gasphase glycine conformers. Chemical Physics Letters, 291(12), 110120.More infoAbstract: The OH vibrational band has been theoretically studied for the two most stable glycine conformers utilizing our variable effective mass approach. The stretching frequencies of the OH bond of the most stable gasphase glycine conformers were predicted. Analytical expressions for the Hamiltonian matrix elements of the vibrational problem were derived. A functional fit to the potential energy term was obtained based on the electronic energies calculated at different OH bond distances at the MP2 level of theory. The calculated OH stretching frequencies are in good agreement with the lowtemperature IR spectroscopic data.
 Borowicz, P., FaurskovNielsen, O., Christensen, D. H., Adamowicz, L., Leś, A., & Waluk, J. (1998). Vibrational spectroscopy of hydroxyheterobiaryls I. Low frequency modes. Spectrochimica Acta  Part A: Molecular and Biomolecular Spectroscopy, 54(10), 12911305.More infoAbstract: Vibrational structure of four molecules known to undergo an extremely rapid excited state proton transfer: [2,2′bipyridyl]3,3′diol, 5,5′dimethyl[2,2′bipyridyl]3,3'diol, [2,2′bipyridyl]3ol and 2(2pyridyl)phenol was studied with FTIR and Raman spectroscopy and ab initio quantum chemical calculations. The assignments for all the observed vibrations lying below 600 cm1 were proposed, based on the comparison of experimental and computational results of transition energies, shifts upon deuterium and methyl substitution, and the analysis of the evolution of individual bands along the series. The calculations appear to be very reliable in predicting the vibrational frequencies, and in reproducing frequency shifts resulting from deuteration and methylation. The assignment of lowfrequency modes may be helpful in understanding of the phototautomerization mechanism, as well as in interpretation of the complicated structure of the band corresponding to the OH stretching vibration. © 1998 Elsevier Science B.V. All rights reserved.
 Desfrançois, C., Périquet, V., Carles, S., Schermann, J. P., & Adamowicz, L. (1998). Neutral and negativelycharged formamide, Nmethylformamide and dimethylformamide clusters. Chemical Physics, 239(13), 475483.More infoAbstract: The structure and low energy electron attachment properties of small clusters of formamide, Nmethylformamide and dimethylformamide have been studied. The experimental technique used is Rydberg electron transfer (RET) between laserexcited atoms and the molecular systems. The richness of hydrogen bonding possibilities of these three highly polar molecules leads to very different structures and resultant dipoles or quadrupoles which we determine by means of an empirical model. A corresponding wide variety of cluster anions is observed and interpreted, ranging from dipolebound or quadrupolebound anions in their ground state or first excited state to valencetype anions.
 Jackowski, K., Leś, A., Da̧mbska, A., & Adamowicz, L. (1998). GIAOCHF and experimental study of the substituent effects on ^{13}C magnetic shielding in benzene derivatives. Polish Journal of Chemistry, 72(7 SUPPL.), 16241629.More infoAbstract: Ab initio calculations of the 13C shielding constants were performed with the use of the GIAOCHF method for the benzene molecule and its 14 monosubstituted derivatives. The theoretical substituent effects were compared with our experimental data which were measured in cyclohexane solutions and extrapolated to infinite dilution. Satisfactory correlations between theoretical and experimental results were observed for all the aromatic carbons except those in the meta position.
 Jarmuła, A., Anulewicz, R., Leś, A., Cyrański, M. K., Adamowicz, L., Bretner, M., Felczak, K., Kulikowski, T., Krygowski, T. M., & Rode, W. (1998). Crystal structures of 5fluorodUrd and its 2 and/or 4thio analogues: Models of substituted dUMP pyrimidine ring interacting with thymidylate synthase. Biochimica et Biophysica Acta  Protein Structure and Molecular Enzymology, 1382(2), 277286.More infoPMID: 9540799;Abstract: In order to understand the influence on thymidylate synthase interactions with dUMP analogues of the pyrimidine ring 2 and/or 4thio, and 5fluoro substitutions, Xray diffractions by crystals of 5fluorodUrd and its 2 and 4thio, and 2,4dithio analogues were measured, the four structures solved and refined. The following conclusions were suggested by results of comparative analyses of structural parameters (bond lengths, valence angles), followed by theoretical considerations based on calculated resonance structure distributions and aromaticity indices of the uracil, thiouracil, fluorouracil and fluorothiouracil rings. The effect of 4thio substitution of FdUMP, altering specificity of inactivation of thymidylate synthases from various sources, is probably due to weaker proton acceptor power of the 4thio substituent and increasing acidity (enhanced protondonor power) of the N(3)H moiety, resulting in an impaired fitness into the network of hydrogen bonds in the enzyme active center cleft. 2,4Dithio substitution results in (i) impaired pyrimidine ring recognition by the enzyme active center, due to the 4thio substituent (ii) increased pyrimidine ring aromaticity in dUMP, leading to resistance of C(6) to nucleophilic attack by the enzyme active center cysteine and (iii) altered planarity of the pyrimidine ring and deflections, with respect to the ring plane, of substituents at C(2), C(4) and C(5). 5Fluoro substitution apparently activates the pyrimidine ring towards the interaction with thymidylate synthase by producing local strain, which results in an increased reactivity as predicted by the WalshBent rule.
 Leś, A., Adamowicz, L., & Rode, W. (1998). Modeling of reaction steps relevant to deoxyuridylate (dUMP) enzymatic methylation and thymidylate synthase mechanismbased inhibition. Journal of Biomolecular Structure and Dynamics, 15(4), 703715.More infoPMID: 9514247;Abstract: Theoretical quantum mechanical ab initio HartreeFock calculations on molecular systems, modeling processes related to the specificity of thymidylate synthase inactivation are reported. We considered several steps of the methylation of the substrate dUMP and 4 or 5mono and 4,5 bisubstituted dUMP analogs, as well. The following reactions were modeled: the cysteine residue (Cys198 in the L. casei enzyme) nucleophilic attack on the substrate and the substrate C(5)H proton abstraction. The substrate was modeled by the 1methyluracil molecule and its structural analogs. The cysteine Cys198 residue was modeled by the methylmercaptane molecule. The substrateenzyme binary complex was modeled by the 1methyl5,6dihydro6 thiomethyluracil (P1) molecule. The present theoretical calculations suggest that the cysteine nucleophilic attack on the substrate may result in the SH group addition to the pyrimidine C(5)=C(6) bond in the course of a weakly exothermic reaction. The formerly presumed enolate carbanion appeared to be weakly stable or unstable and it can readily split into the thiol and pyrimidine residues. The s2thio (P2) and s2,4dithio (P3) substrate analogs should form stable thiolate anions after cysteine residue attachment to the C(6) position of the pyrimidine ring. Studies of the deformed P1 molecule interacting with a water molecule bound to the pyrimidine C(4)=O carbonyl residue allow a suggestion that this water molecule may be directly involved in the C(5)H proton abstraction and may serve as a proton transmitter between the substrate and the proton acceptor residue, possibly located on the cofactor N10nitrogen. Interaction of the pyrimidine C(4)=O group, or its modification, with the N5,10methylenetetrahydrofolate N(10) nitrogen atom is suggested as an additional factor influencing the inhibition process.
 M., D., Smets, J., Elkadi, Y., & Adamowicz, L. (1998). Ab initio theoretical study of dipolebound anions of molecular complexes: Water tetramer anions. Journal of Chemical Physics, 109(4), 12381244.More infoAbstract: We present results of ab initio calculations of the (H2O)4/(H2O)4 system. The main conclusions of this work are as follows: The calculated results predict that water tetramer anions are metastable systems in agreement with weak spectral manifestation of these systems in gasphase experiments of Bowen and coworkers; the excess electrons in all four structural isomers of water tetramer anions found in the calculations are attached to the clusters by the virtue of dipoleelectron interaction; all four (H2O)4 anions found in the calculations are almost isoenergetic but have different vertical electron detachment energies (VDEs) ranging from 22 to 279 meV; the most stable cyclic structure of (H2O)4 has a null dipole moment and does not form a dipolebound state with an excess electron; the water tetramer anions observed experimentally probably are formed as a result of hydration of the water dimer anion, (H2O)2, by a neutral water dimer or by hydration of the water trimer anion, (H2O)3, by a single water molecule; based on calculations, we predict some specific IR vibrational features for the anions which can be used for identification of these species; vibrational analysis of all four anions found in the calculations indicate that they correspond to minima on the potentialenergy surface. © 1998 American Institute of Physics.
 McCarthy, W. J., Lapinski, L., Nowak, M. J., & Adamowicz, L. (1998). Outofplane vibrations of NH_{2} in 2aminopyrimidine and formamide. Journal of Chemical Physics, 108(24), 1011610128.More infoAbstract: The outofplane vibrations of the amino group in primary amines involve its inversion and rotation relative to the rest of the molecule. These two vibrations in 2aminopyrimidine (see Fig. 1) were investigated with the combined use of matrixisolation IR spectroscopy and ab initio quantum mechanical methodology. The ab initio methodology was also applied to another primary amine, formamide, for which a gasphase IR spectra have been previously assigned. Ab initio potential energy surfaces were calculated in internal coordinates, ω and τ, whose displacements mimicked the inversion and internal rotation normalmode distortions of the amino group, respectively. Vibrations along these two coordinates were considered uncoupled from all other nuclear motions. Total energy was calculated at the secondorder MøllerPlesset perturbation theory level at selected values of ω and τ to allow a leastsquares fitting of an analytical function depicting the potential energy curves and surface. A numerical procedure for determining the values of the kinetic energy operator in internal coordinates was also implemented to which an analytical function was fit. Vibrational energy expectation values were variationally determined by utilizing products of Gaussian and sinusoidal functions as the basis set. The resultant calculated fundamental transition energies for the coupled inversion and internal rotation vibrations of 2aminopyrimidine are vinv=140.6 cm1 and vrot=440.3 cm1, respectively. These theoretical values reasonably match the experimental quantities of v≈200 cm1 and v≈500 cm1, and allow firm assignment of these two experimental infrared spectral bands to the inversion and internal rotation vibrations of the amino group in 2aminopyrimidine, respectively. For formamide (see Fig. 2), the calculated transition energies for the inversion and internal rotation vibrations, vinv=249 cm1 and vrot=602, match the experimental frequencies of ≈289 cm1 and ≈602 cm1, and confirm the accuracy of the theoretical method. © 1998 American Institute of Physics.
 McCarthy, W. J., Smith, D. M., Adamowicz, L., SaintMartin, H., & OrtegaBlake, I. (1998). An ab initio study of the isomerization of Mg and Capyrophosphates. Journal of the American Chemical Society, 120(24), 61136120.More infoAbstract: Ab initio calculations were performed on gasphase calcium and magnesium dications chelated with various anionic pyrophosphate species: H2P2O72, HP2O73 and P2O74. The cleavage of the pyrophosphate into a metaphosphate and an orthophosphate complexed to either calcium or magnesium was also investigated. The studied isomerization reaction of the metalpyrophosphate complexes can be written as [M·H(N)P2O7](N2) → [PO3·M·H(N)PO4](N2) where M = Mg, Ca, and N = 0, 1, 2. Geometries for the complexes were optimized with the selfconsistentfield (SCF) level of theory, and the total energy for each system was subsequently calculated with the secondorder MollerPlesset perturbation (MP2) method using 631+G** basis functions for the H, O, and P atoms and valence doubleζ basis functions polarization augmented with a diffuse function (pVDZ+) for the Mg and Ca atoms. Zeropoint energies (ZPE) and entropies were calculated with the SCF harmonic frequencies from which enthalpies and Gibbs free energies were also estimated. The ab initio isomerization energies of all of the calciumcontaining complexes were positive and had a large contribution lower contribution of correlation. These calculated gasphase isomerization energies may provide an explanation to the observation that pyrophosphatases utilize magnesium complexes as substrates for the hydrolysis of pyrophosphates but do not utilize calcium complexes.
 Rostkowska, H., Nowak, M. J., Lapinski, L., & Adamowicz, L. (1998). Molecular structure and infrared spectra of 2hydroxy1,4naphthoquinone; Experimental matrix isolation and theoretical HartreeFock and post HartreeFock study. Spectrochimica Acta  Part A: Molecular and Biomolecular Spectroscopy, 54(8), 10911103.More infoAbstract: The infrared spectra of 2hydroxy1,4naphthoquinone (HNQ) isolated in lowtemperature Ar and N2 matrices are reported. The calculations of electronic energies for possible tautomers of HNQ (performed at RHF/631 + + G** and MP2/631 + + G** levels of theory) and the comparison of the experimental spectra with those theoretically calculated (at the RHF/631 + + G** and DFT(B3LYP)/631 + + G** levels) show that the molecules of the compound isolated in lowtemperature matrices exist in the form with intramolecular hydrogen bond. Theoretical results reproduce well the experimental infrared spectra of the matrix isolated HNQ. The assignment of the infrared absorption bands due to monomers of the compound isolated in Ar and N2 matrices to the theoretically predicted normal modes is presented. © 1998 Elsevier Science B.V. All rights reserved.
 Schoone, K., Smets, J., Houben, L., K., M., Adamowicz, L., & Maes, G. (1998). Matrixisolation FTIR studies and theoretical calculations of hydrogenbonded complexes of molecules modeling adenine tautomers. 1. Hbonding of benzimidazoles with H_{2}O in ar matrices. Journal of Physical Chemistry A, 102(25), 48634877.More infoAbstract: This work opens a series of studies on the water complexes of adenines. We use a similar approach as used in our earlier studies of cytosinewater complexes (i.e., first we investigate the IR spectral manifestations of hydrogenbonding at selected interaction sites of the stable amino N9H tautomeric form of adenine by studying simpler model molecules which have only a single or a very few selected hydrogenbond interaction sites typical for adenine). The present study concerns the first two of such model molecules, benzimidazole and 1CH3benzimidazole. IR vibrational spectra of matrixisolated benzimidazole, 1CH3benzimidazole, and their complexes with water are analyzed and assigned by comparing the experimental spectra with the IR frequencies and intensities computed with the use of ab initio and density functional theory (DFT) methods. When the DFT/B3LYP/631++G** monomer frequencies are scaled with three different scaling factors, the mean differences between the experimental and calculated frequencies are only 10 and 8 cm1 for benzimidazole and 1CH3benzimidazole, respectively. The calculated, MP2/631++G**//RHF/631++G** (MP2 denotes the secondorder MøllerPlesset Perturbation Theory, RHF denotes the restricted HartreeFock method, and notation MP2//RHF denotes that the molecular geometries were optimized at the RHF level and then used to calculate total energies using the MP2 method), Hbond interaction energies, with the basis set superposition error accounted for, are 22.6, 21.2, and 22.0 kJ/mol for the benzimidazole N1H⋯OH2 and N3⋯HOH complexes and the 1CH3benzimidazole N3⋯HOH complex, respectively. The DFT/B3LYP/631++G** method yields similar Hbond interaction energies. The frequency shifts of the vibrational modes directly involved in the Hbond interactions are better predicted by the DFT method than by the RHF method. For other vibrational modes not directly involved in the Hbonds, the two methods provide a similar level of accuracy in predicting the shifts of the fundamental modes caused by Hbonding interactions. In this work we also establish correlations between experimental and theoretical characteristics of the NH⋯OH2 Hbonding in water complexes of benzimidazole and 1CH3benzimidazole, and these correlations will be used in future elucidation of FTIR spectra of water complexes of adenine.
 Smets, J., Destexhe, A., Adamowicz, L., & Maes, G. (1998). Matrixisolation FTIR studies and ab initio calculations of hydrogenbonded complexes of molecules modeling cytosine or isocytosine tautomers. 6. Experimental observation of a waterinduced tautomeric shift for 2hydroxypyrimidine and 5bromo2hydroxypyrimidine. Journal of Physical Chemistry A, 102(42), 81578168.More infoAbstract: The Hbond interaction of the cytosine model compound 2hydroxypyrimidine and its 5bromo derivative with water is investigated using the combined matrixisolation FTIR and theoretical ab initio method. As predicted by the ab initio calculations, both compounds occur dominantly in the hydroxy tautomeric forms. The estimated KT(h/o) values are 60 and 184, respectively. When water is added to the Ar matrix, a noticeable shift of the tautomeric equilibrium towards the oxo form is observed. The theoretical results indicate that the closed N⋯HO(H)⋯HO and C=O⋯HO(H)⋯HN Hbonded water complexes are the most stable systems for the hydroxy and the oxo tautomers, respectively. The experimental spectra are consistent with this prediction, but additional structures, such as an open N⋯HOH complex of the hydroxy tautomer, are also identified. The frequency shift of the stretching mode of doubly Hbonded water in the two closed complexes is larger, and the ratio between the calculated and measured frequencies smaller than expected from the correlation established before for open, singly Hbonded complexes involving similar molecules. Although some cooperativity exists between the two H bonds in each of the closed complexes, this effect is limited because the geometrical structures of both H bonds are noticeably perturbed from the perfect alignment due to the cyclic arrangement of the complex. A possible mechanism of the proton transfer process leading from the hydroxy to the oxo tautomeric form is discussed in terms of proton tunneling and in relation to recent literature data.
 Smets, J., Schoone, K., Ramaekers, R., Adamowicz, L., & Maes, G. (1998). Theoretical density functional and ab initio computational study of the tautomeric, vibrational and Hbond properties of 1,7diCH_{3}guanine: A comparison with FTIR data from matrix isolation experiments. Journal of Molecular Structure, 442(13), 201220.More infoAbstract: In this work we report the results of a combined matrixisolation FTIR and theoretical computational DFT and ab initio study of the tautomeric and vibrational characteristics of 1,7dimethylguanine (17DMG). The HF, MP2 and DFT methods are all in agreement in predicting that the aminooxo tautomer of 17DMG is the most stable form, and that the iminooxo tautomer, which can exist in two different geometrical isomers, is the second most stable form. The energy difference between the aminooxo and aminooxo tautomer is rather large: 23, 25 and 33 kJ mol1, for the HF, DFT and MP2 methods, respectively. The iminohydroxy tautomer is, according to the calculations, by far the least stable form (ΔE > 180 kJ mol1). The FTIR spectra of 17DMG isolated in an argon matrix are in agreement with the theoretical calculation indicating that only the aminooxo tautomer should be present in the gas phase in detectable amounts. The experimental IR frequencies agree quite well with the theoretical values. When a single scaling factor is applied to approximately correct the calculated frequencies for various systematic errors in theoretical approach, the mean frequency deviations for the HF/631++G**, DFT /631G** and DFT/631++G** calculated spectra are 17.5 cm1, 12.2 cm1 and 16.3 cm1, respectively. The DFT meanfrequencydeviations decrease to 8.1 cm1 and 10.1 cm1 when variable factors are applied. The stabilities of the different water complexes of the aminooxo tautomer of 17DMG are investigated by optimizing the complexes at the HF/631++G** level of theory and calculating the MP2 energies at these optimized geometries. The N3...HOH complex, which has a close structure with two Hbonds, is predicted to be the most stable structure. The MP2 calculated energy differences between the most stable complex and the C6=O...HOH, the HNH...OH2 and the H2N...HOH heterodimers are 14.28 kJ mol1, 1954 kJ mol1 and 20.73 kJ mol1, respectively.
 Smets, J., Smith, D. M., & Adamowicz, L. (1998). Ab initio theoretical study of dipolebound anions of molecular complexes. Anions of Hbonded hydrogen cyanide polymers. Chemical Physics Letters, 297(56), 451457.More infoAbstract: Ab initio calculations have been performed to determine the relative stabilities and structures of small hydrogen cyanide polymers and their dipolebound anions. It is predicted that while the formation of cyclic structures should dominate in the neutral environment, in the freeelectronrich environment linear clusters should be preferably formed.
 Smets, J., Smith, D. M., Elkadi, Y., & Adamowicz, L. (1998). Ab initio theoretical study of dipolebound anions of molecular complexes. Water molecule inhibits or enhances electron affinity of Nmethylaminoadenine. Polish Journal of Chemistry, 72(7 SUPPL.), 16151623.More infoAbstract: Ab initio calculations have been performed to determine the electron affinity of the water complex of Nmethylaminoadenine (NMA). This complex has been chosen for this study because it can form three different isomeric structures with a water molecule hydrogenbonded at three different NMA sites, and only two of these structures have sufficient dipole moments to form dipolebound states with an excess electron. In the third structure, which according to the calculations should be the most stable form, the dipole moments of NMA and the water molecule oppose each other resulting in an almost null dipole moment of the complex, so the complex has no ability to form a stable dipolebound anion. This determination offers an interesting possibility for an experimental investigation of whether the NMA.H2O complex forms an stable anion. If such an anion is detected, it will mean that an electron attachment can increase the gas phase concentration of the complex, which is thermodynamically less stable in its neutral form.
 Smith, D. M., Smets, J., Elkadi, Y., & Adamowicz, L. (1998). Ab initio theoretical study of dipolebound anions of molecular complexes. [H _{2} O ⋯ HCN] ^{} and [HCN ⋯ H _{2} O] ^{} anions. Chemical Physics Letters, 288(56), 609615.More infoAbstract: Ab initio calculations have been performed to determine structures and adiabatic electron affinities of water complexes of hydrogen cyanide. In these systems the excess electron is bound by the dipole field of the complex. Based on the calculations we determined that, as for the neutral complexes, there are two equilibrium anion structures, [HCN⋯H2O] and [H2O⋯HCN]. Upon electron attachment to the neutral H2O⋯HCN complex, which the only system detected in the gas phase, its stability with respect to the HCN⋯H2O complex increases even further, from 1.4 kcal/mol to 3.0 kcal/mol. The adiabatic electron affinities of H2O⋯HCN and HCN⋯H2O complexes are predicted to be 86 and 17 meV, respectively.
 Stepanian, S. G., Reva, I. D., Radchenko, E. D., & Adamowicz, L. (1998). Conformational behavior of αalanine. Matrixisolation infrared and theoretical DFT and ab initio study. Journal of Physical Chemistry A, 102(24), 46234629.More infoAbstract: Two conformers of the nonionized αalanine and its isotopomer N,N,Od3alanine have been observed in lowtemperature Ar matrixes. Their infrared spectra have been analyzed and assigned using DFT/B3LYP/ augccpVDZ and MP2/augccpVDZ geometry and frequency theoretical calculations. Two different intramolecular Hbonds, bifurcated NH2⋯O=C and N⋯HO, were found in the observed αalanine conformers, I and IIa. We found that the DFT/B3LYP/augccpVDZ method yields vibrational frequencies of the αalanine conformers in excellent agreement with the experimental data.
 Stepanian, S. G., Reva, I. D., Radchenko, E. D., Rosado, M. T., Duarte, M. L., Fausto, R., & Adamowicz, L. (1998). Matrixisolation infrared and theoretical studies of the glycine conformers. Journal of Physical Chemistry A, 102(6), 10411054.More infoAbstract: The IR spectra of nonionized glycine and its deuterated derivatives isolated in the lowtemperature argon matrices have been measured, and for the first time the infrared spectral characteristics of the three most stable conformers have been determined and assigned. Correlated level ab initio and density functional theory (DFT) calculations of IR frequencies and intensities with extended basis sets were performed and their results were employed to separate the bands of the glycine conformers in the experimental spectra and to assist the assignment. The intramolecular interconversion, conformer III → conformer I, which is observed in the matrices at temperatures higher than 13 K, was found to cause a significant decrease of the band intensities of conformer III in the spectra. This phenomenon was used to distinguish the vibrational bands of this conformer from the bands of the other conformers. The reliability of the MøllerPlesset secondorder perturbation theory (MP2) method and the DFT method with the threeparameter density functional, Becke3LYP, in the prediction of the IR spectra of the nonionized glycine conformers was examined. We found that the DFT/Becke3LYP method with augccpVDZ basis set yields vibrational frequencies of the glycine conformers very similar to the MP2 results. Both DFT and MP2 results are in the excellent agreement with the experimental data.
 Borowicz, P., FaurskovNielsen, O., Christensen, D. H., Adamowicz, L., Leś, A., & Waluk, J. (1997). Vibrational spectroscopy of hydroxyheterobiaryls. Low frequency modes of 2(2hydroxyphenyl)3pyridynol. Journal of Molecular Structure, 408409, 363366.More infoAbstract: Influence of intermolecular hydrogen bonding upon the vibrational pattern in the lowfrequency region was studied for 2(2hydroxyphenyl)3pyridynol a molecule which reveals both intra and intermolecular hydrogen bonds in the crystal phase. Combined use of IR and Raman spectroscopy and ab initio 631G** calculations for the isolated and watercomplexed compound made it possible to find low frequency transitions corresponding to the vibrations of the water solvate.
 Gilmore, D. W., Kozlowski, P. M., Kinghorn, D. B., & Adamowicz, L. (1997). Analytic first derivatives for explicitly correlated, multicenter, Gaussian geminals. International Journal of Quantum Chemistry, 63(5), 991999.More infoAbstract: Variational calculations utilizing the analytic gradient of explicitly correlated Gaussian molecular integrals are presented for the ground state of the hydrogen molecule. Preliminary results serve to motivate the need for general formulas for analytic first derivatives of molecular integrals involving multicenter, explicitly correlated Gaussian geminals with respect to Gaussian exponents and coordinates of the orbital centers. Explicit formulas for analytic first derivatives of Gaussian functions containing correlation factors of the form exp(βrii2) are derived and discussed. © 1997 John Wiley & Sons, Inc.
 Houben, L., Schoone, K., Smets, J., Adamowicz, L., & Maes, G. (1997). Combined matrixisolation FTIR and abinitio 631++G** studies on tautomeric properties of nucleic acid bases and simpler model molecules. Journal of Molecular Structure, 410411, 397401.More infoAbstract: The combined matrix isolation/ab initio 631++G** method is used to study the tautomeric equilibria of purine, 1CH3adenine, 3CH3adenine and NCH3adenine in isolated conditions. For the N9H ⇆ N7H prototropism, tautomeric equilibrium constants of 0.085, 0.090 and 0.005 are obtained for purine, 1CH3adenine and NCH3adenine, respectively. The other prototropic equilibria that are considered are the amino ⇆ imino tautomerism for which relative amounts of 5.5% for the rare amino form in 1CH3adenine and 0.5% for the minor imino tautomer of 3CH3 adenine were observed, and the N9H ⇆ N3H prototropism where only 0.7% of the experimental spectrum could be accounted for by the presence of the less stable N3H form in NCH3adenine.
 K., M., Smets, J., Schoone, K., Houben, L., McCarthy, W., Adamowicz, L., Nowak, M. J., & Maes, G. (1997). Matrixisolation FTIR studies and theoretical calculations of hydrogenbonded complexes of imidazole. A comparison between experimental results and different calculation methods. Journal of Physical Chemistry A, 101(13), 23972413.More infoAbstract: The hydrogen bond interaction between water and imidazole was investigated with the matrixisolation FTIR spectroscopy coupled to ab initio calculations performed with the RHF and MP2 methods and the parametrized DFT method with the B3LYP hybrid functional. The 631G** and 631++G** basis sets were used in the calculations. Evaluation of the accuracy of the three methods and the two basis sets was made for noncomplexed imidazole. All three of the methods gave geometries for imidazole in good agreement with the experimental structure. Also, all three levels of theory with both basis sets gave similarly accurate vibrational frequency predictions for monomeric imidazole with a best mean deviation for the DFT/B3LYP/ 631++G** method. The assignment of the matrix spectra of the two isomeric Hbond complex species, NH⋯OH 2 and N⋯HOH, was performed by comparison with the theoretically predicted IR frequencies and intensities and was further assisted by asymmetrical deuteration experiments. The MP2 and DFT methods employed with the basis set augmented with diffuse functions gave good predictions of the frequency shifts for the vibrational modes directly influenced by the Hbond interaction. For the other vibrational modes, the RHF method performed almost as equally well as the MP2 and DFT methods and we can conclude that this method can provide qualitative and quantitively reliable data on hydrogenbonded systems.
 Kinghorn, D. B., & Adamowicz, L. (1997). A new Nbody potential and basis set for adiabatic and nonadiabatic variational energy calculations. Journal of Chemical Physics, 106(21), 87608768.More infoAbstract: A new functional form for multibody expansions of potential energy surfaces and basis functions for correlated adiabatic and fully nonadiabatic variational energy calculations is presented. Nbody explicitly correlated Gaussians with premultiplying factors consisting of products of powers of internal distance coordinates are utilized in a dual role to analytically represent isotropic potentials and energy eigenfunctions in the same internal coordinate system. Practical aspects of this new methodology are presented. The ideas and methods are prototyped and illustrated with two simple diatomic examples; the Morse potential and an accurate H2 potential for which essentially exact results are obtained for vibrational energy levels. © 1997 American Institute of Physics.
 Kinghorn, D. B., & Adamowicz, L. (1997). Electron affinity of hydrogen, deuterium, and tritium: A nonadiabatic variational calculation using explicitly correlated Gaussian basis functions. Journal of Chemical Physics, 106(11), 45894595.More infoAbstract: Nonadiabatic variational calculations for the anion ground state energies, mass shifts, and electron affinities of Hydrogen, Deuterium and Tritium are reported. Electron affinities values are 6083.0994, 6086.7137, and 6087.9168 cm1 for Hydrogen, Deuterium, and Tritium, respectively. These results were obtained using a basis of explicitly correlated Gaussians. Exact nonrelativistic energy bounds are carefully predicted: E(H) = 0.5 274 458 811, E(D) = 0.5 275 983 247, and E(T) = 0.5 276 490 482 in hartree energy units. It is shown that these new bounds cannot be obtained using the infinite nuclear mass approximation plus Rydberg scaling and the usual first order mass polarization correction. © 1997 American Institute of Physics.
 M., D., Smets, J., Elkadi, Y., & Adamowicz, L. (1997). Ab initio theoretical study of dipolebound anions of molecular complexes. Water trimer anion. Journal of Chemical Physics, 107(15), 57885793.More infoAbstract: We present results of ab initio calculations of our study of (H2O)3. The main conclusions of this work are as follows: the most stable cyclic structure of (H2O)3 has a dipole moment too small to form a dipolebound state with an excess electron; the dipolebound anion of the water trimer observed experimentally appears to be a hydrated complex of the water dimer anion, (H2O)2, by a single water molecule. The water trimer anion, (H2O)3, has an open shape. The calculated vertical electron detachment energy of this anion is predicted to be equal to 141 meV, which is in good agreement with the experimental value of Bowen and coworkers, equal to 142±7 meV. Although the open optimal geometry of the (H2O)3 anion obtained in the present calculations is an equilibrium structure, its energy is higher than the energy of the cyclic equilibrium structure of the neutral complex, indicating that the anion is a metastable system. Based on calculations, we predict significant differences in the IR vibrational spectra of (H2O)3 and (H2O)3, which may be used for identification of the two species. © 1997 American Institute of Physics.
 M., D., Smets, J., Elkadi, Y., & Adamowicz, L. (1997). Methylation reduces electron affinity of Uracil. Ab initio theoretical study. Journal of Physical Chemistry A, 101(43), 81238127.More infoAbstract: Following the experimental characterization of the N,Ndimethylated uracil anion by Bowen and coworkers, we have undertaken an investigation of the influence of the methylation on the electron affinity of the uracil molecule. Both experiment and theory agree that, as it is in the case of the isolated uracil molecule, the methylated uracils can only attach excess electrons into diffuse dipolebound states. The corresponding electron affinities are very small (several MeV). The bonding effect in the dipolebound state depends on the magnitude of the molecular dipole and on the size of the molecule. Selective methylation of the uracil molecule can be used to reduce or increase the dipole value and to change the electron affinity of the molecule. The present calculated results are consistent with the experimental determination that N,Ndimethylation of uracil results in reduction of the electron affinity.
 Maes, G., Smets, J., Adamowicz, L., McCarthy, W., Bael, M. V., Houben, L., & Schoone, K. (1997). Correlations between ab initio and experimental data for isolated Hbonded complexes of water with nitrogen bases. Journal of Molecular Structure, 410411, 315322.More infoAbstract: Correlations between selected ab initio predicted and experimentally observed properties of 1:1 Hbonded complexes of pyridines, pyrimidines, and imidazoles with water are investigated. Relationships are found between the experimental properties of proton affinity and water frequency shift, and the ab initio calculated bond distances, interaction energies and water frequency shifts. It is also found that welldefined relations can be established between calculated and observed properties for the pyridine complexes, but these cannot be reliably extended to the other Nbase systems. The similarities demonstrate that the presently available ab initio methods are useful in predicting the experimental behaviour of Hbonded systems, but only for closely related molecules.
 McCarthy, W., Plokhotnichenko, A. M., Radchenko, E. D., Smets, J., Smith, D. M., Stepanian, S. G., & Adamowicz, L. (1997). Hbonded and stacked dimers of pyrimidine and pbenzoquinone. A combined matrix isolation infrared and theoretical ab initio study. Journal of Physical Chemistry A, 101(39), 72087216.More infoAbstract: Matrix isolation IR spectroscopy and highlevel ab initio calculations were applied to investigate the structure and vibrational spectra of quinonepyrimidine heterodimers formed in lowtemperature Ar matrices. A specially developed experimental technique was used to separate bands of quinonepyrimidine dimer from bands of quinone and pyrimidine monomers and homodimers in the IR spectra. As a result, nine bands assigned to the quinonepyrimidine heterodimer were identified. Ab initio calculations at the MP2/631+G*, MP2/631++G** and SCF/631++G** levels of theory have been carried out to determine the relative energies and vibrational spectra of three stable configurations of the quinonepyrimidine dimer found theoretically. These configurations are two planar complexes with two weak CH⋯O and CH⋯N hydrogen bonds and one stacked complex stabilized by dispersion forces. The effect of basis set superposition error (BSSE) on the relative stabilities and the vibrational spectra of the dimers was also investigated. The nonBSSEcorrected calculations at the MP2/631+G* and MP2/631++G** levels of theory predict the stacked dimer to be the most stable conformer, but accounting for BSSE resulted in a reverse stability ordering of the stacked and the planar dimers. The comparison of the observed frequency shifts with the theoretically predicted shifts has shown that the planar configuration is responsible for the experimentally observed bands. This is in agreement with the stability ordering derived from the BSSEcorrected relative energies. To account for the matrix effects on the stability of the planar and stacked dimers, additional calculations were carried out using the Onsager's reaction field model and the MP2/631++G** level of theory. These calculations confirm that the planar Hbonded dimer is the most stable configuration.
 Mccarthy, W., Smets, J., Adamowicz, L., Plokhotnichenko, A. M., Radchenko, E. D., Sheina, G. G., & Stepanian, S. G. (1997). Competition between Hbonded and stacked dimers of pyrimidine: IR and theoretical abinitio study. Molecular Physics, 91(3), 513525.More infoAbstract: A combined matrix isolation Fourier transform infrared experimental and abinitio theoretical approach has been used to examine the vibrational spectra of pyrimidine dimers. It was found that the presence of pyrimidine dimers resulted in experimentally observed frequencies shifted relative to those of the pyrimidine monomer. Four stable pyrimidine dimer conformers were found via abinitio calculations. Two of these conformers namely a stacked geometry and a Hbonded geometry in which pyrimidine monomer dipole moments orient antiparallel, had the lowest but very similar energies. Abinitio harmonic frequencies of these two dimers and their shifts relative to the monomer's values were determined. The effect of Basis Set Superposition error on these calculated frequency shifts was also investigated. The comparison of the abinitio predicted and experimentally observed frequency shifts and an accompanying potential energy distribution analysis suggest that the lowestenergy Hbonded configuration is responsible for the experimentally observed phenomena.
 Ramaekers, R., M., D., Elkadi, Y., & Adamowicz, L. (1997). Dipolebound anion of hydrogen fluoride dimer: Theoretical ab initio study. Chemical Physics Letters, 277(13), 269274.More infoAbstract: Ab initio calculations have been performed to determine the electron affinity of the hydrogen fluoride dimer, (HF)2. Although, a single hydrogen fluoride molecule does not form a stable stationary state with an excess electron, the hydrogen fluoride dimer has a positive electron affinity. In this system the excess electron is bound by the dipole field of the complex. The present theoretical calculations render the value of adiabatic electron affinity equal to 52 meV. The calculated value is in a good agreement with experimental result of 63 ± 6 meV of Bowen and coworkers. © 1997 Elsevier Science B.V.
 Ramaekers, R., M., D., Smets, J., & Adamowicz, L. (1997). Ab initio theoretical study of dipolebound anions of molecular complexes: (HF)_{3}^{} and (HF)_{4}^{} anions. Journal of Chemical Physics, 107(22), 94759481.More infoAbstract: Ab initio calculations have been performed to determine structures and vertical electron detachment energy (VDE) of the hydrogen fluoride trimer and tetramer anions, (HF)3 and (HF)4. In these systems the excess electron is bound by the dipole field of the complex. It was determined that, unlike the neutral complexes which prefer the cyclic structures, the equilibrium geometries of the anions have "zigzag" shapes. For both complexes the predicted VDEs are positive [210 meV and 363 meV for (HF)3 and (HF)4, respectively], indicating that the anions are stable systems with respect to the vertical electron detachment. These results were obtained at the coupledcluster level of theory with single, double and triple excitations [CCSD(T) method; the tripleexcitation contribution in this method is calculated approximately using the perturbation approach] with the anion geometries obtained using the secondorder MøllerPlesset perturbation theory (MP2) method. The same approach was also used to determine the adiabatic electron affinities (AEA) of (HF)3 and (HF)4. In addition to the electronic contribution, we also calculated the contributions (using the harmonic approximation) resulting from different zeropoint vibration energies of the neutral and anionic clusters. The calculations predicted that while the AEA of (HF)3 is positive (44 meV), the AEA for (HF)4 is marginally negative (16 meV). This suggests that the (HF)3 anion should be a stable system, while the (HF)4 is probably metastable. © 1997 American Institute of Physics.
 Rostkowska, H., Nowak, M. J., Lapinski, L., Smith, D., & Adamowicz, L. (1997). Molecular structure and infrared spectra of 3,4dihydroxy3cyclobutene1,2dione; experimental matrix isolation and theoretical HartreeFock and post HartreeFock study. Spectrochimica Acta  Part A: Molecular and Biomolecular Spectroscopy, 53(7), 959968.More infoAbstract: The infrared spectra of 3,4dihydroxy3cyclobutene1,2dione (squaric acid) isolated in lowtemperature Ar and N2 matrices are reported. The comparison of the experimental spectra with those theoretically calculated at HF/631 + + G**, MP2/631G** and DFT(B3LYP)/631 + +G** levels shows that the molecules of the compound isolated in lowtemperature matrix adopt the structure with C2v symmetry. This structure is different from the one which was found in the crystal. Theoretical calculations of relative energies of C2v and Cs structures predict the C2v conformation to be lower in energy by 8 kJ mol1. Good agreement between the experimental and theoretical spectra allowed for a reliable assignment of the observed IR absorption bands. © 1997 Elsevier Science B.V.
 Slanina, Z., Lee, S., & Adamowicz, L. (1997). C _{80}, C _{86}, C _{88}: Semiempirical and ab initio SCF calculations. International Journal of Quantum Chemistry, 63(2), 529535.More infoAbstract: Three complete sels of the isolatedpentagonrule fullerene isomers are consideredC 80 (7 species), C 86 (19 species), and C 88 (35 species). The interisomerk separation energetics is essentially similar at semiempirical (AMI, SAM1) and ab initio (HF/STO3G, HF/321G, HF/431G) levels. For the C 0 set, the HF/431G computations explain why the lowestenergy isomer is not experimentally observed. The computations also agree with observations on C 86 and C 88. © 1997 John Wiley & Sons, Inc. J Quant Chem.
 Slanina, Z., Uhlik, F., & Adamowicz, L. (1997). An analytical solution of isotopomeric enumerations for C(2v) cyclic oddnumbered carbon clusters C(n). Journal of Radioanalytical and Nuclear Chemistry, 219(1), 6971.More infoAbstract: The current experimental and computational research in carbon clusters pointed out a need for enumeration of all possible 12C/13C isotopomers of the C(2v) cyclic oddnumbered carbon clusters C(n). It is shown that the number of the isotopomers is given by 2(k1)(2(k1) + 1) with k=(n + 1)/2. Some illustrative computed structure and energy data for the clusters are reported, too.
 Smets, J., Destexhe, A., Adamowicz, L., & Maes, G. (1997). Matrixisolation FTIR studies and ab initio calculations of hydrogenbonded complexes of molecules modeling cytosine or isocytosine tautomers. 4. Hbonded complexes of 1methyl2pyrimidone and N,N1trimethylcytosine with water. Journal of Physical Chemistry B, 101(33), 65836599.More infoAbstract: We continue our combined experimental and theoretical ab initio studies of hydrogenbonded complexes involving molecules modeling cytosines by investigating the Hbond interaction of 1methyl2pyrimidone and N,N1trimethylcytosine with water. The objective of the present work is theoretical and spectral characterization of the basewater complexes in the argon matrix, where the water molecule can become hydrogen bonded to the base through two H bonds with hydrogens of both bonds coming from the water molecule. For the 1methyl2pyrimidone base, the ab initio calculations predict the Hbonded complex at the N 3 site to be slightly more stable than the alternative complex at the C 2=O site. Both Hbonded complexes have a second, weaker H bond between the other water OH group and the C 2=O or N 3 base site. The predicted vibrational spectra of the two complexes do not match well with the observed experimental FTIR spectrum. Possible reasons for the discrepancies between the experiment and the theory attributed to the presence of argon are discussed. While the theory predicts two closed complexes, each containing a double hydrogen bond, from the experimental spectrum, two open complexes at the C 2=O and N 3 interaction sites are identified. Hbonding of water with N,N,1trimethylcytosine can occur at either the N 3, C 2=O, or (CH 3) 2N site. The stability difference between the first two complexes is relatively small, and the second H bond in both complexes is considerably weaker than in the pyrimidone complexes. As results from the calculations, the Hbond interaction at the methylated amino group is much weaker than the H bonds formed with N 3 and C 2=O, and this finds conformation in the experimental matrix spectra, where the spectral signature of this complex is absent. The relative frequency shifts and the ratios between the calculated and measured frequency values for the stretching mode of the bonded water for all the complexes of both basis are discussed in relation to the earlier established correlations for similar Hbonded complexes. Waterrich matrices contain also 1:2 complexes B⋯HO(H)⋯HOH. It is found that the central H bond in these linear structures is characterized by the cooperativity factor of 1.31.4.
 Smets, J., M., D., Elkadi, Y., & Adamowicz, L. (1997). Search for stable anions of uracilwater clusters. Ab initio theoretical studies. Journal of Physical Chemistry A, 101(48), 91529156.More infoAbstract: In this work we investigate the ability of the uracil·water complex to form stable anionic systems. As the experimental evidence and theoretical calculations have indicated, the isolated uracil molecule can only attach an excess electron into a diffuse dipolebound state, while some recent experiments suggest that the uracil· water complex can form a more stable valencetype anion. In this work we demonstrate that it is possible to converge ab initio calculations of uracil·(H2O)3 to an equilibrium structure that is significantly different from the structure of the neutral cluster and that has a positive and remarkably significant vertical ionization potential. Apart from the valence anion, the uracil·(H2O)3 complex can form a stable dipole  bound anion, but as the present calculations indicate the electron affinity, which corresponds to this attachment, is very small (13 meV). The structure of the dipolebound anion is virtually identical with the structure of the neutral complex.
 Adamowicz, L., & Malrieu, J. (1996). Muitireference selfconsistent sizeextensive stateselective configuration interaction. Journal of Chemical Physics, 105(20), 92409247.More infoAbstract: In this work, we propose a statespecific selfconsistent "dressing" of the multireference configuration interaction (MRCI) space to include all single and doublesubstituted determinants for the most important reference configurations. The aim of the method is to provide a sizeextensive description of the dynamic electron correlation effects for states which mandate a multideterminantal reference wave function. Such states can represent electronic excited states or ground states of the molecular systems which are significantly deformed from their equilibrium structures. The proposed approach follows the concept introduced in our recently proposed quasilinear ansatz for the stateselective multireference coupledcluster method. The purpose of the dressing procedure is to eliminate the contributions which introduce sizeextensivity violating terms in the MRCI approach. © 1996 American Institute of Physics.
 Adamowicz, L., Caballol, R., Malrieu, J. P., & Meller, J. (1996). A general bridge between configuration interaction and coupledcluster methods: A multistate solution. Chemical Physics Letters, 259(56), 619626.More infoAbstract: Using the concept of intermediate Hamiltonians, a statespecific dressing of an arbitrary configuration interaction matrix is presented in its full generality, proceeding through a coupledcluster development of the desired eigenvector from its dominant single determinantal components. The method is then generalized to simultaneously search for M eigenstates through a JeziorskiMonkhorsttype expansion from the M dominant determinants.
 Elkadi, Y., & Adamowicz, L. (1996). Dipolebound anions of ethylene glycol dimers. Theoretical ab initio study. Chemical Physics Letters, 261(45), 507514.More infoAbstract: Ab initio calculations have been performed to determine the electron affinity of the ethylene glycol molecule and its dimer. Although, as determined, the glycol molecule has only a marginal ability to bind an excess electron, the three glycol Hbonded dimers, whose structures were found in the calculations, have much higher electron affinity values. In all cases the excess electron is bound by the dipole field of the complex. The attachment of the excess electron lowers the relative energy differences between the three complexes, making their coexistence more probable.
 Ghose, K. B., Piecuch, P., Pal, S., & Adamowicz, L. (1996). Stateselective multireference coupledcluster theory: In pursuit of property calculation. Journal of Chemical Physics, 104(17), 65826589.More infoAbstract: In this work, we examine the efficiency of the recently developed [P. Piecuch et al., J. Chem. Phys. 99, 6732 (1993)] stateselective (SS) multireference (MR) coupledcluster (CC) method for calculation of molecular properties. In our earlier papers, we demonstrated that the SSMRCC method with inclusion of single, double, and internal and semiinternal triple excitations [SSCCSD(T) approach] is capable of providing an accurate description of the groundstate potential energy surfaces. In this paper, we present the dipole moment and polarizability values of the HF molecule at equilibrium and stretched geometries calculated using finite field technique and SSCCSD(T) ansatz. The calculations use double zeta quality basis sets with and without polarization functions. Molecular orbital basis sets include both relaxed and nonrelaxed orbitals. © 1996 American Institute of Physics.
 Kozlowski, P. M., & Adamowicz, L. (1996). Lifetime of positronium molecule. study with boys' explicitly correlated gaussians. Journal of Physical Chemistry, 100(15), 62666271.More infoAbstract: Nonadiabatic variational calculations for the positronium molecule, Ps2 = (e+e )i, a cluster composed of two electrons and two positrons, is the subject of theoretical investigation. In particular, calculations of the twophoton annihilation rate as well as estimation of the lifetime at the nonrelativistic level of theory are presented in detail. In all calculations the nonadiabatic fourbody wave function is expanded in terms of explicitly correlated Gaussian functions. The energy of the centerofmass motion is effectively eliminated from the total nonrelativistic energy through the modified variational principle based on the internal Hamiltonian. With the wave function expanded in terms of 300 basis functions, which leads to a total energy equal to 0.515 980 au and a binding energy equal to 0.435 eV, the lifetime of the system is estimated to be 0.907 ns. © 1996 American Chemical Society.
 Lapinski, L., Prusinowska, D., Nowak, M. J., Bretner, M., Felczak, K., Maes, G., & Adamowicz, L. (1996). Infrared spectra of 6azathiouracils: An experimental matrix isolation and theoretical ab initio SCF/6311G study. Spectrochimica Acta  Part A: Molecular and Biomolecular Spectroscopy, 52(6), 645659.More infoAbstract: The infrared (IR) absorption spectra of 2thio6azauracil, 4thio6azauracil and 2,4dithio6azauracil isolated in lowtemperature Ar and N2 matrices are reported. These compounds have been found to exist in lowtemperature matrices exclusively in the oxothione and dithione tautomeric forms. An assignment of the observed infrared bands is proposed on the basis of comparison of the experimental matrix spectra with the spectra calculated at the ab initio SCF/6311G** level. The IR spectra of the title compounds are compared with the previously published IR spectra of matrix isolated 2thiouracil, 4thiouracil and 2,4dithiouracil. © 1996 Elsevier Science B.V. All rights reserved.
 Schoone, K., Houben, L., Smets, J., Adamowicz, L., & Maes, G. (1996). Matrixisolation FTIR and ab initio 631 + + G** study of 1CH_{3}adenine tautomerism. Spectrochimica Acta  Part A Molecular Spectroscopy, 52(4), 383396.More infoAbstract: The vibrational spectrum of 1CH3adenine was studied by the combined experimental matrixisolation and theoretical ab initio SCF/631 + +G ** method. The imino N9H tautomer is largely predominant in Ar matrices in accordance with the theoretical energy predictions. Trace amounts of the imino N7H and the amino tautomers are detected for the first time, and tautomerization constants of 0.09 and 0.05 were obtained from a combination of the experimental and the theoretical IR intensities. The results are discussed in relation to the prototropic behaviour of adenine and purine and to the mechanism of the tautomerization process.
 Smets, J., Adamowicz, L., & Maes, G. (1996). Matrixisolation FTIR studies and ab initio calculations of hydrogenbonded complexes of molecules modeling cytosine or isocytosine tautomers. 5. 1CH_{3}cytosine complexes with H_{2}O in Ar matrices. Journal of Physical Chemistry, 100(16), 64346444.More infoAbstract: The combined experimental and theoretical ab initio method for investigation of hydrogenbonded complexes between water and molecules modeling cytosines has been applied to 1CH3cytosine. This compound occurs in Ar predominantly in the aminooxo form, but traces of the iminooxo E tautomer are also present in accordance with the ab initio prediction. An approximate value of 7.2 is obtained for the tautomerization constant KT(ao/io) using the infrared intensity measurement. The most stable Hbonded complex of the abundant ao tautomer with water is the closed Na3⋯HO⋯N14 structure, which shows cooperativity between the two Hbonds. Less stable complexes are the closed N3⋯HOH⋯O7 complex and the open N14H⋯OH2 and N14⋯HOH structures. In the experimental FTIR spectra, the closed N3⋯HO⋯HN14 and open O7⋯HOH complexes are identified based on characteristic absorptions predicted by ab initio calculations or based on earlier obtained experimental results for similar structures. Despite some trace amounts of the io E tautomer in the Ar matrix, Hbonding of this rare form with water was detected in the form of the closed N14⋯HO⋯HN3 structure. A preliminary correlation between the frequency shifts of the bonded water stretching mode and the proton affinities of the C=O group is presented for the open C=O⋯HOH complexes of 1CH3cytosine and some related compounds. © 1996 American Chemical Society.
 Smets, J., Houben, L., Schoone, K., Maes, G., & Adamowicz, L. (1996). Multiple site proton affinities of methylated nucleic acid bases. Chemical Physics Letters, 262(6), 789796.More infoAbstract: Proton affinities of nucleic acid base molecules methylated at the site where the sugar connects to the base in the DNA helix are calculated by the use of ab initio methods. Geometry optimizations and frequency calculations are performed at the HF/631G(d) level, while the final energies and proton affinities are calculated with the MP2/6311(2d,2p) method. The proton affinities are reported for temperatures of 0 and 298.15 K. The proton affinity values for the most basic protonation sites are: 891.5 kJ/mol for 1methyluracil, 982.8 kj/mol for 1methylcytosine, 961.3 kJ/mol for 9methyladenine and 980.5 kJ/mol for 9methylguanine. The calculated proton affinity values of the singly methylated nucleic acid bases are in agreement with the experimental values for the sugarylated bases, the nucleosides.
 Smets, J., McCarthy, W. J., & Adamowicz, L. (1996). Dipolebound electron attachment to uracilwater complexes. Theoretical ab Initio study. Journal of Physical Chemistry, 100(35), 1465514660.More infoAbstract: Ab initio calculations performed in this work found positive electron affinities for all three possible doubly Hbonded complexes of the uracil molecule with a single water molecule. In all cases the excess electron is bound by the the dipole field of the complex. No conventional stable "valence" anionic states were found with the theoretical procedure used in this work (SCF + secondorder perturbation theory corrections for the electron correlation effects). The attachment of the excess electron lowers the relative energy differences between the three complexes, making their coexistence more probable. Structural changes in the uracilwater complex upon attachment of an electron were also found. The anion's equilibrium geometry had noticeably shortened hydrogenbond lengths and a shifted orientation of the water molecule with respect to the uracil molecule compared to the neutral system. © 1996 American Chemical Society.
 Smets, J., McCarthy, W. J., & Adamowicz, L. (1996). Water molecule enhances dipolebound electron affinity of 1methylcytosine. Chemical Physics Letters, 256(45), 360369.More infoAbstract: We investigate electron attachment to water complexes of 1methylcytosine. In all the systems considered the excess electron is bonded to the dipole field of the complex. The calculations show that in the cases of the aminooxo tautomer with a water molecule Hbonded to the amino group and with a water molecule Hbonded to the carbonyl group, enlargement of the dipole moment due to alignment of the dipoles of the water and base molecules leads to an almost fourfold increase of the electron affinity in comparison to the isolated 1methylcytosine molecule.
 Sobolewski, A. L., & Adamowicz, L. (1996). An ab initio study of the potential energy surface in the S^{1} state of 2hydroxypyridine. Chemical Physics, 213(13), 193201.More infoAbstract: The potential energy (PE) surface of the lowest excited singlet state relevant to the photophysics of interconversion of the 2hydroxypyridine/2(1H)pyridinone (2HP/2PY) system is characterized by ab initio calculations. The geometry optimizations in the S1 state performed at the CIS/631G(d,p) level confirm the experimental findings that the 2HP form is planar in this state whereas the 2PY form is significantly outofplane distorted. The lowestenergy firstorder saddlepoints which are relevant to the photophysics of the "mobile" hydrogen atom on the S1 PE surface lead toward its dissociation from the molecule. There is no saddlepoint for a "direct" proton transfer reaction on the S1 PE surface leading from the 2HP form to the 2PY form. The results confirm the hypothesis that the photoinduced dissociationassociation (PIDA) mechanism is probably responsible for the excitedstate tautomerization observed in this system.
 Sobolewski, A. L., & Adamowicz, L. (1996). Doubleprotontransfer in [2,2′bipyridine]3,3′diol: An ab initio study. Chemical Physics Letters, 252(12), 3341.More infoAbstract: The intramolecular twoproton transfer reaction in the ground and lowest excited states has been characterized for the model molecule of [2,2′bipyridine]3,3′diol (BP(OH)2) with the use of ab initio techniques. In accord with the experimental findings it has been determined that only a single tautomeric form of BP(OH)2 exists in the ground state (dienol form) and in the lowest excited single state (diketo form), respectively. The excitedstate proton transfer reaction occurs on an essentially barrierless potential energy surface. There is a significant barrier for the proton transfer reaction on the lowest excited triplet state surface.
 Sobolewski, A. L., & Adamowicz, L. (1996). Photophysics of 2hydroxypyridine: An ab Initio study. Journal of Physical Chemistry, 100(10), 39333941.More infoAbstract: The potential energy (PE) surfaces of the electronic ground and the lowest excited states relevant to photophysics of interconversion of the 2hydroxypyridine/2(1H)pyridone system are characterized by ab initio calculations. The energy calculations at the optimized geometries are performed with the aid of the secondorder perturbation theory, employing the completeactivespace selfconsistentfield (CASSCF) wave function as the reference (CASPT2). Results confirm the earlier hypothesis based on the CASSCF calculations (Chem. Phys. Lett. 1993, 211, 293) that the photoinduced dissociationassociation (PIDA) mechanism is probably responsible for excitedstate tautomerization observed in this system. Absorption of the second photon in the excited electronic state is suggested to provide the "driving" force for the PIDA mechanism. © 1996 American Chemical Society.
 Alexandrov, V., Piecuch, P., & Adamowicz, L. (1995). Stateselective multireference coupledcluster theory employing the singlereference formalism: Application to an excited state of H_{8}. The Journal of Chemical Physics, 102(8), 33013306.More infoAbstract: The stateselective (SS) multireference (MR) coupledcluster (CC) method exploiting the singlereference (SR) formalism [P. Piecuch, N. Oliphant, and L. Adamowicz, J. Chem. Phys. 99, 1875 (1993)] is applied to the first excited totally symmetric singlet state of a prototype molecular system composed of eight hydrogen atoms. Minimum basis set is employed and various geometries are considered. The SS CC energies are compared with the results of the stateuniversal (SU) MR CC calculations involving single and double excitations (CCSD) as well as with the exact energies obtained using the full configuration interaction method. Comparison is also made with the results of the standard SR CCSD calculations. In both nondegenerate and quasidegenerate regions, our SS CC theory truncated at double excitations [SS CCSD(TQ) method] provides much better description of the first excited state than the genuine multideterminantal SU CCSD formalism. © 1995 American Institute of Physics.
 Borowicz, P., Grabowska, A., Kaczmarek, Ł., Leś, A., & Adamowicz, L. (1995). Synthesis, photophysics and theoretical ab initio calculations of a bizwitterionic compound modeling the phototautomer of bipyridyldiol. Chemical Physics Letters, 239(46), 282289.More infoAbstract: A new compound, 3hydroxy1,1′dimethyl3oxido[2,2′bipyridinedium]hydroxide hydrate (1,1′dimethyl3,3′dioxido[2,2′bipyridinedium]  DDB), was synthesized as a model of the transient product of the excited state intramolecular proton transfer (ESIPT) reaction (excited phototautomer) of bipyridyldiol (BP(OH)2). The photophysics and quantum chemical ab initio calculations of DDB are reported. The results confirm the zwitterionic character of the excited phototautomer of BP(OH)2. © 1995.
 Buyl, F., Smets, J., Maes, G., & Adamowicz, L. (1995). Matrixisolation FTIR studies and abinitio calculations of hydrogenbonded complexes of molecules modeling cytosine or isocytosine tautomers. 3. Complexes of 4hydroxypyridine and 3hydroxypyridine with H_{2}O in Ar matrices. Journal of Physical Chemistry, 99(41), 1496714979.More infoAbstract: The hydrogenbond interaction of 4hydroxypyridine and 3hydroxypyridine with H2O is investigated with the use of a combined experimental matrixisolation FTIR and theoretical abinitio method. Both of these isomeric compounds occur in the hydroxy form in lowtemperature Ar matrices, in accordance with the predicted stability difference from the oxo form of 22 and 55 kJ/mol for 4hydroxypyridine and 3hydroxypyridine, respectively. The most stable Hbonded complex of the hydroxy tautomers of both compounds with water is the OH⋯OH2 structure, and the stability differences with the nearest lying isomeric N⋯HOH structures are 6.74 and 6.41 kJ/mol, respectively. Both complexes are identified with the use of the predicted frequency perturbations. The correlation between the predicted interaction energies and the frequency shifts of the modes of bonded water and the scaling factors of these modes is analyzed for the series pyrimidine, pyridine, 3hydroxypyridine, 4hydroxypyridine, and 4aminopyridine. The relative basicity and acidity of 4hydroxypyridine and 3hydroxypyridine are estimated from this analysis and discussed. © 1995 American Chemical Society.
 Ghose, K. B., & Adamowicz, L. (1995). Use of recursively generated intermediates in state selective multireference coupledcluster method: A numerical example. The Journal of Chemical Physics, 103(21), 93249330.More infoAbstract: The present work represents the first attempt to utilize the idea of recursively generated intermediates (RGI) in the framework of the stateselective multireference coupledcluster method truncated at triple excitations [SS CCSD(T)]. The expressions for stepwise generation of intermediates are so structured that the spin and point symmetry simplifications can be easily applied during computation. Suitable modifications in SS CCSD(T) equations are introduced to allow for optional quasilinearization of nonlinear terms in difficult convergence situations. The computational code is, as expected, much faster than the SS CCSD(T) code without RGI adaptation. This has been numerically demonstrated by potential energy surface (PES) calculation of the HF molecule using a double zeta basis. © 1995 American Institute of Physics.
 Ghose, K. B., Piecuch, P., & Adamowicz, L. (1995). Improved computational strategy for the stateselective coupledcluster theory with semiinternal triexcited clusters: Potential energy surface of the HF molecule. The Journal of Chemical Physics, 103(21), 93319346.More infoAbstract: The recently developed stateselective (SS) multireference coupledcluster (CC) method involving all singly and doubly, and semiinternal triply excited clusters from the formal reference configuration [SSCCSD(T) approach] is tested in the calculation of the potential energy surface (PES) of the HF molecule. Both double zeta and double zeta plus polarization basis sets are employed and a few different choices of active space are considered. The SSCCSD(T) method provides an accurate description of the entire PES at low cost even for the bond breaking region, contrary to the results obtained with the perturbative singlereference CCSD(T) method or various limited configuration interaction approaches. This is the first application of the new SSCC code, which uses an improved computational strategy for handling the semiinternal triexcited clusters. Details of this new implementation of the SSCCSD(T) method are discussed. © 1995 American Institute of Physics.
 Gutsev, G. L., & Adamowicz, L. (1995). Electronic and geometrical structure of dipolebound anions formed by polar molecules. Journal of physical chemistry, 99(36), 1341213421.More infoAbstract: The electronic and geometrical structures of fourteen polar molecules, which have sufficiently large dipole moments to form dipolebound stationary states with an extra electron, are investigated with the HartreeFock (HF) method and secondorder MollerPlesset (MP2) perturbation theory. The molecules considered are as follow: formaldehyde, H2CO; propanal, CH3CH2CHO; pivalaldehyde, (CH3)3CCHO; butanal, CH3CH2CH3CHO; acetaldehyde, CH3CHO; 2butanone, CH3CH2COCH3; trifluorobenzene, C7H5F3; cyclohexanone, C6H10O; acetone; CH3COCH3; cyclopentanone, C5H8O; cyclobutanone, C4H6O; methylacrylonitrile, CH2CCH3CN; acrylonitrile, CH2CHCN; acetonitrile, CH3CN. The electron affinities corresponding to the formation of the dipolebound states of the anions are calculated. For all the molecules considered, except formaldehyde, the molecules are found to be able to support dipolebound states. Almost direct proportionality is found between the dipole moment and the electron affinity (EA) in the series.
 Gutsev, G. L., & Adamowicz, L. (1995). Relationship between the dipole moments and the electron affinities for some polar organic molecules. Chemical Physics Letters, 235(34), 377381.More infoAbstract: The electronic and geometrical structures of fourteen polar molecules are investigated with the HartreeFock (HF) method and the secondorder MøllerPlesset (MP2) perturbation theory. The molecules are: formaldehyde, H2CO, propanol, CH3CH2CHO, pivalaldehyde, (CH3)3CCHO, butanal, CH3CH2CH2CHO, acetaldehyde, CH3CHO, 2butanone, CH3CH2COCH3, trifluoromethylbenzene, C7H5F3, cyclohexanone, C6H10O, acetone, CH3COCH3, cyclopentanone, C5H8O, cyclobutanone, C4H6O, methylacrylonitrile, CH2CCH3CN, acrylonitrile, CH2CHCN, acetonitrile, CH3CN. The electron affinities corresponding to the formation of the dipolebound states of the anions are calculated. For all the molecules considered, except formaldehyde, the molecules are found to be able to support dipolebound states. © 1995.
 Gutsev, G. L., & Adamowicz, L. (1995). The structure of the CF_{4}^{} anion and the electron affinity of the CF_{4} molecule. The Journal of Chemical Physics, 102(23), 93099314.More infoAbstract: The electronic and geometrical structure of the CF4 anion and its neutral parent, CF4, are calculated with the secondorder MollerPlesset perturbation theory. Several diffuse sp shells were added to the standard 631+G* basis when calculating the potential energy surface of the CF4+e system. It was found that the CF4 molecule does not attach an additional electron in the ground state, i.e., the molecule possesses a zero vertical electron affinity under the BornOppenheimer approximation. The optimized C3v and C2v configurations of the anion are transition states, whereas its Cs configuration corresponds to a local minimum and is thermodynamically stable by 20 kcal/ mol. The CF4 molecule has the negative adiabatic electron affinity of 1.22 eV with respect to this configuration of the anion. © 1995 American Institute of Physics.
 Gutsev, G. L., & Adamowicz, L. (1995). The valence and dipolebound states of the cyanomethide ion, CH_{2}CN^{}. Chemical Physics Letters, 246(3), 245250.More infoAbstract: The electronic and geometrical structures of the cyanomethyl radical, CH2CN, and cyanomethide ion, CH2CN, are investigated with the HartreeFock method, MøllerPlesset perturbation theory and coupled cluster theory. It is found that the lowest singlet and triplet dipolebound states (1B1 and 3B1) of the anion correspond to a σattachment of the extra electron to the ground (2B1, C2v state of the CH2CN radical. The ground valence (1A′, Cs) anionic state results from a πattachment, which leads to some pyramidalization of the flat configuration of the neutral parent. The calculated electron affinities of CH2CN with respect to both anionic states are in good agreement with the experimental data. © 1995.
 Gutsev, G. L., Sobolewski, A. L., & Adamowicz, L. (1995). A theoretical study on the structure of acetonitrile (CH_{3}CN) and its anion CH_{3}CN^{}. Chemical Physics, 196(12), 111.More infoAbstract: The electronic and geometrical structures of the CH3CN molecule and its anion CH3CN are calculated at the UHF, UMP2, UMP4, CASSCF and CASPT2 levels of theory. The anion CH3CN is confirmed to possess a dipolebound state. The electron affinity (EA) of CH3CN is computed to be ≈ 6 meV. The potential energy surfaces (PES) of both the neutral and anionic species are computed as functions of the distance between carbon atoms of the CH3 and CN constituents. It is found that the neutral molecule possesses the avoiding crossing between two lowest singlet states having different dissociation limits, namely CH3(doublet) + CN(doublet) and CH3+(singlet) + CN(singlet). The anionic PES is always lower than the neutral one and possesses an appreciable barrier of ≈ 0.8 eV at the distance range of 2.02.5 Å between the CH3 and CN groups. This barrier separates the dipolebounded and "normal" bound states of the CH3CN anion. After passing the barrier there is a local minimum at the anion PES with respect to which the adiabatic EA of the neutral CH3CN molecule is negative of ≈ 1.6 eV. © 1995.
 Lapinski, L., Nowak, M. J., Leś, A., & Adamowicz, L. (1995). Comparison of ab initio HF/631G^{**}, HF/631 + + G^{**} and MP2/631G^{**} calculated infrared spectra of 4(3H)pyrimidinone and 4hydroxypyrimidine with matrix isolation spectra. Vibrational Spectroscopy, 8(3), 331342.More infoAbstract: The results of ab initio calculations of infrared (IR) spectra of 4hydroxypyrimidine and 4(3H)pyrimidinone are reported. The vibrational spectra computed at the HF/631G**, HF/631 + + G** and MP2/631G** levels of theory were found to reproduce well the experimental IR spectra of the Ar matrix isolated species. However, neither the enlargement of the basis set, with respect to the 631G** set, nor partial account for the electronic correlation effects at the MP2 level were found to provide any considerable improvement of the predictions of the IR spectra. © 1995.
 McCarthy, W. J., Lapinski, L., Nowak, M. J., & Adamowicz, L. (1995). Anharmonic contributions to the inversion vibration in 2aminopyrimidine. The Journal of Chemical Physics, 103(2), 656662.More infoAbstract: The outofplane vibrations of the amino group in 2aminopyrimidine have large amplitudes, and cannot be properly described within the harmonic approximation. The normal mode analysis carried out at this level of approximation at the restricted HartreeFock level and at the secondorder MøllerPlesset perturbation theory level failed to match the experimental transition frequency of ν≈200 cm1 of the inversion vibration in this compound. In an effort to better understand this vibration motion, we went beyond the harmonic approximation. The inversion vibration was treated as being uncoupled from all other nuclear degrees of freedom. An internal coordinate (ω) was chosen whose displacement mimicked the outofplane distortion of the amino group during the inversion vibration. Electronic energy was calculated at the secondorder MøllerPlesset perturbation theory level at selected values of ω to form a doublewell curve describing a model potential within which the nuclei move during the vibration. This potential was incorporated into a onedimensional Hamiltonian, and vibrational energy expectation values were variationally determined by utilizing the harmonic wavefunctions as the basis set. Two sets of calculations were performed: one in which the mirror plane of symmetry was preserved throughout the vibrational deformation limiting the internal coordinates to 17, and another in which the symmetry was unconstrained permitting description by 3N6=30 internal coordinates. These calculations resulted in prediction of the ν=0→ν=1 transition energy of ν=130.1 cm1 and ν=206.7 cm1, respectively, reasonably matching the experimental value of ≈200 cm1. © 1995 American Institute of Physics.
 Piecuch, P., & Adamowicz, L. (1995). Breaking bonds with the stateselective multireference coupledcluster method employing the singlereference formalism. The Journal of Chemical Physics, 102(2), 898904.More infoAbstract: The stateselective (SS) multireference coupledcluster (CC) method exploiting the singlereference (SR) formalism [P. Piecuch, N. Oliphant, and L. Adamowicz, J. Chem. Phys. 99, 1875 (1993)] is applied to BH and H2O at equilibrium and displaced geometries. Different selections of active spaces are considered. Comparison with the SR CC approaches involving single and double (CCSD), single, double, and triple (CCSDT) and single, double, triple, and quadruple (CCSDTQ) excitations, and with the full configuration interaction method, indicates remarkable stability and accuracy of the SS CC results for difficult bond breaking cases. For the first time, the ab initio SS CC calculations include the most complete SS CCSD(TQ) approximation, which emerges through selection of the most essential clusters appearing in the full SR CCSDTQ theory. © 1995 American Institute of Physics.
 Prusinowska, D., Lapinski, L., Nowak, M. J., & Adamowicz, L. (1995). Tautomerism, phototautomerism and infrared spectra of matrixisolated 2quinolinethione. Spectrochimica Acta Part A: Molecular Spectroscopy, 51(11), 18091826.More infoAbstract: The IR spectra of thione and thiol tautomers of 2quinolinethione isolated in Ar and N2 lowtemperature matrices are reported. The UVinduced photochemical reaction, transforming the initially abundant thione form into the thiol form, was used for separation of the spectra of the two tautomers. The ratio of the tautomers before UV irradiation of the sample was estimated as [thione]/[thiol] = 7:1. The experimental spectra of 2(1 H)quinolinethione and 2quinolinethiol were interpreted by comparison with the spectra calculated theoretically at the ab initio SCF/631 G** level. © 1995.
 Roehrig, G. H., Oyler, N. A., & Adamowicz, L. (1995). Dipolebound excesselectron states of adenine tautomers. A theoretical ab initio study. Journal of physical chemistry, 99(39), 1428514289.More infoAbstract: Ab initio calculations performed in this work render a nearzero but positive electron affinity of the major N(9)H tautomer of adenine. However, the electron affinity of the N(7)H tautomer, which has not been detected in the gas phase but coexists with the N(9)H tautomer in the polar solution, is predicted to be equal to 0.12 eV. ln both cases the excess electron is bonded to the dipole field of the molecule. No conventional stable `valence' anionic states have been found. Due to a significant difference between the predicted electron affinities of the N(7)H and N(9)H tautomers, one should expect a noticeable shift in the tautomeric equilibrium when electrons attach to adenine molecules in the gas phase.
 Slanina, Z., Lee, S. ., Smigel, M., Kurtz, J., & Adamowicz, L. (1995). Smaller carbon clusters: linear, cyclic, polyhedral. Materials Research Society Symposium  Proceedings, 359, 163168.More infoAbstract: The MP2 perturbation treatment with the 631G basis set has been applied to linear, cyclic, and polyhedral structures of Cn, n = 613, esp. to C12 in this report. While the linear and cyclic species can coexist, polyhedral ones are too high in energy. Preliminary results on C8H (related to diffuse interstellar bands) are reported, too.
 Slanina, Z., Sun, M. ., Lee, S. ., & Adamowicz, L. (1995). Fullerene neighbours. Materials Research Society Symposium  Proceedings, 359, 175180.More infoAbstract: Semiempirical quantumchemical calculations are reported for the fullerenic structures C60, Si60, Ge60, N60; B36N24, B36P24, Al36N24, Al36P24; and various BnNn. A new route towards B/N clusters is considered, being based on squares and hexagons. The pattern always requires six squares. The route can produce species of similar or even higher stability comparing to the conventional pentagon/hexagon pattern. Four particular stoichiometries emerge from the available AM1 computations: B12N12, B28N28, B36N36, and B36N24.
 Smets, J., Adamowicz, L., & Maes, G. (1995). Matrixisolation FTIR studies and abinitio calculations of hydrogenbonded complexes of molecules modeling cytosine or isocytosine tautomers. 2. 4Aminopyridine and 4aminopyrimidine complexes with H_{2}O in Ar matrices. Journal of Physical Chemistry, 99(17), 63876400.More infoAbstract: Hbonding interactions of the cytosinemodeling compounds 4aminopyridine and 4aminopyrimidine with water are investigated with the use of a combined experimental matrixisolation FTIR and theoretical abinitio method. Both bases appear exclusively as the amino tautomers in lowtemperature Ar matrices, in accordance with large abinitio predicted energy differences with the rare imino forms. The most stable complex of water with 4aminopyridine is the Hbonded N1⋯HOH structure, but the energy difference with the alternative NH⋯OH2 complex is calculated to be only 6.7 kJ/mol. Both complexes are observed in the Ar matrix. In the case of 4aminopyrimidine, the closed NH⋯OH⋯N3 structure containing two Hbonds is found to be significantly more stable than the singly Hbonded structures N1⋯HOH and NH⋯OH2 by 8.0 and 10.2 kJ/mol, respectively. The previously suggested relation between the scaling factor for the theoreticallycalculated vbOH mode of the Hbonded water and the proton affinity value of the protonacceptor site in the base is developed further. © 1995 American Chemical Society.
 Sobolewski, A. L., & Adamowicz, L. (1995). Ab initio characterization of electronically excited states in highly unsaturated hydrocarbons. The Journal of Chemical Physics, 102(1), 394399.More infoAbstract: The results of extensive electronic structure calculations performed for a series of molecules: (CC)nH, H(CC)nH+ for n=2,3,4 and for (CC)3H and H(CC)3H are presented. The theoretical scheme is based on the completeactivespace selfconsistentfield (CASSCF) method, with remaining dynamic electron correlation effects added in the subsequent step with the use of the secondorder perturbation theory with the CASSCF wave function as the reference state (CASPT2). The results support the hypothesis of Fulara et al [Nature 336, 439 (1993)] that the 2Π→2Π electronic transitions in the series of CnH radicals might be responsible for some of the "diffuse interstellar bands" observed in the visible and nearinfrared spectra of stars located on the far side of diffuse interstellar clouds. © 1995 American Institute of Physics.
 Sobolewski, A. L., & Adamowicz, L. (1995). Theoretical investigations of proton transfer reactions in a hydrogen bonded complex of cytosine with water. The Journal of Chemical Physics, 102(14), 57085718.More infoAbstract: The potential energy functions of the electronic ground as well as the lowest ππ* and nπ* excited singlet states of the most stable hydrogen bonded complex of cytosine with water was theoretically investigated along the proton transfer (PT) reaction coordinate. The full geometry optimization was performed along the PT reaction path. In the geometry optimization, the HartreeFock method and the configuration interaction scheme with single excitations was used. The energy calculations at the optimized geometries were performed with the completeactivespace selfconsistentfield (CASSCF) method and with secondorder perturbation theory, employing the CASSCF wave function as the reference state (CASPT2), as well as with the MollerPlesset secondorder theory (MP2) for the ground state. It was found that the cytosine:water complex is stable in the aminooxo form against the PT reaction which leads to the iminooxo tautomeric form on the ground as well as on the excited PE surfaces. An efficient nonradiative decay channel of electronically excited cytosine resulting from nonadiabatic interactions between the ground and excited singlet states along the reaction coordinate leading to the prefulvenic form was identified. It is argued that both the above mentioned effects under the normal conditions (neutral aqueous solution, room temperature) are responsible for remarkable stability of cytosine in its "native" aminooxo form against any damage that may result from exposure to UV radiation. © 1995 American Institute of Physics.
 Sobolewski, A. L., & Adamowicz, L. (1995). Theoretical investigations of the excitedstate intramolecular proton transfer reaction in Nsubstituted3hydroxypyridinones. Chemical Physics, 193(12), 6778.More infoAbstract: The potential energy functions of the electronic ground state, as well as the lowest nπ* and ππ* excited singlet states of 3hydroxy4pyridinone and 3hydroxy2methyl4pyridinone, have been theoretically investigated along the proton transfer (PT) reaction coordinate. The full geometry optimization has been performed along the PT reaction path. In the geometry optimization, the HartreeFock approximation and the configuration interaction scheme with single excitations have been employed. The energy calculations at the optimized geometries have been performed with the completeactive space selfconsistent field (CASSCF) method followed by secondorder perturbation theory calculations, employing the CASSCF wave function as the reference. We found that the neardegeneracy between the ππ* and nπ* excited singlet states, which is removed upon methyl substitution, might be the factor which differentiates the two systems with respect to the excited state intramolecular PT reaction. A simple vibrational model has been proposed for investigation of the dynamics of the process. © 1995.
 Sobolewski, A. L., & Adamowicz, L. (1995). Theoretical investigations of the proton transfer reaction in hydrogenbonded complexes of cytosine with HNO. Chemical Physics Letters, 234(13), 94100.More infoAbstract: The potential energy (PE) functions of the lowest singlet and triplet states of the hydrogenbonded complexes of cytosine with HNO and NOH were theoretically investigated along the proton transfer (PT) coordinate. A full geometry optimization was performed along the PT reaction path at the HartreeFock level of theory. The energies at the optimized geometries were calculated with the use of secondorder MøllerPlesset perturbation theory (MP2) and with secondorder perturbation theory employing the complete active space selfconsistent field wavefunction as the reference (CASPT2). It was found that the cyclic complex of the 'native' aminooxo form of cytosine with NOH can be exothermally transformed on the barrierless PE surface into the complex of the 'rare' iminooxo form with HNO. The results provide a model of a chemically induced PT reaction in nucleic acid bases which can effectively generate their 'rare' tautomeric forms. © 1995.
 Sobolewski, A. L., & Adamowicz, L. (1995). Theoretical investigations of the proton transfer reaction in the hydrogenbonded complex of 2pyrimidinone with water. Journal of physical chemistry, 99(39), 1427714284.More infoAbstract: The potential energy (PE) functions of the electronic ground and lowest ππ* excited singlet states of the hydrogenbonded cyclic complex of 2pyrimidinone with water were theoretically investigated along the proton transfer (PT) reaction coordinate. The full geometry optimization was performed along the PT reaction path. In the geometry optimization the HartreeFock method and the configuration interaction method with single excitations (CIS) were used. The energy calculations at the optimized geometries were performed with the completeactivespace selfconsistentfield (CASSCF) method and with the secondorder perturbation theory, employing the CASSCF wave function as the reference. For the ground state, calculations were also performed with the MollerPlesset secondorder perturbation theory (MP2). We found that the hydroxy form of the 2pyrimidinone:water complex is stable in the ground electronic state while the hydroxytooxo transformation reaction of the complex is by about 0.67 eV exothermic on the lowest 1 ππ* excited state PE surface. However, there is a barrier of about 0.19 eV along the PT reaction path on this surface. The top of the barrier is below the energy of the vertical excitation So → 1ππ*; thus, the photoexcited system has sufficient excess energy for the PT reaction to occur spontaneously.
 Destexhe, A., Smets, J., Adamowicz, L., & Maes, G. (1994). Matrix isolation FTIR studies and ab initio calculations of hydrogenbonded complexes of molecules modeling cytosine or isocytosine tautomers. 1. Pyridine and pyrimidine complexes with H_{2}O in Ar matrices. Journal of Physical Chemistry, 98(5), 15061514.More infoAbstract: An experimental method is described for a vibrational spectroscopic study of the Hbonding properties of cytosines and isocytosines in lowtemperature inert matrices. The method comprises a detailed experimental and theoretical ab initio study of several molecules of increasing complexity, each modeling one or more interaction sites of cytosine tautomers which have been demonstrated to occur in lowtemperature Ar matrices. A first series of results is presented for the simple Nbases pyridine and pyrimidine complexed with H2O in Ar. Pyridine forms the stronger N⋯H2O complex, which is manifested in relatively far displaced H2O absorptions and in stronger perturbations of the base ring fundamentals. The experimental results are supported by the SCF/631++G** predicted shifts. Two different 1:2 complexes are identified for pyrimidine/H2O in annealed matrices. The presence of N⋯HO and HO⋯HO Hbonds in the first and of N1⋯HO and N3⋯HO Hbonds in the second complex is deduced from strongly different Hbond cooperativities in these complexes. © 1994 American Chemical Society.
 Gutsev, G., Leś, A., & Adamowicz, L. (1994). The electronic and geometrical structure of aluminum fluoride anions AlF_{n}^{}, n=14, and electron affinity of their neutral parents. The Journal of Chemical Physics, 100(12), 89258933.More infoAbstract: The electronic and geometrical structure of AlF, AlF 2, AlF3, and AlF4 as well as their neutral parents are determined at the unrestricted HartreeFock (UHF) and secondorder unrestricted MøllerPlesset (UMP2) levels of theory. The results of the calculations are used for estimating the adiabatic electron affinity (E.A.) of the neutrals and fragmentation energies of both the neutrals and anions. All these characteristics were also recomputed using the UMP2/631+G* geometry at the fourthorder UMP4 level of theory. According to the results of the single, double, triple, and quadrupleexcitation fourthorder MøllerPlesset (MP4SDTQ) calculations, the AlF molecule in the ground state has no positive E.A., AlF2 possesses a rather high E.A. value of 1.90 eV, and AlF3 has a smaller E.A. of 0.93 eV, whereas the last member in the series AlF4 possesses an extremely high E.A. of 7.96 eV. Such a large value is related to high stability of the anion towards fragmentation opposite to the neutral AlF4 whose dissociation energy is computed to be only 45 kcal/mol. It has been predicted that all the anions are rather stable towards detachment of a fluorine anion, and AlF4 is the most stable in the series. It is shown the AlF molecule is able to attach an additional electron at large interatomic distances and form a stable anion. This indicates a possibility of an increasing tendency to form resonances when the molecule is excited into a higher vibrational state. Our calculations also show that AlF in the lowest triplet state may attach an additional electron to form a metastable anion in the quartet state. This anion is predicted to be stable towards both detachment of the extra electron and dissociation. A similar increasing tendency to formation of resonance states with an additional electron with vibrational excitation is also predicted for AlF2. Also in this case, at some stretched geometries, the anion in the lowest triplet state becomes more stable than the neutral system. However, relaxation of its geometry leads to its decay to the neutral in the doublet state plus a free electron. © 1994 American Institute of Physics.
 Lapinski, L., Nowak, M. J., Leś, A., & Adamowicz, L. (1994). Ab initio calculations of IR spectra in identification of products of matrix isolation photochemistry: Dewar form of 4( 3H)pyrimidinone. Journal of the American Chemical Society, 116(4), 14611467.More infoAbstract: Ab initio calculations of the infrared spectrum of the Dewar isomer of 3methyl4(3H)pyrimidinone have been carried out at the MP2/631G** and SCF/631G** levels of theory. These were compared with the experimental spectrum of the photoproduct that emerged upon UV (308 nm) irradiation of 3methyl4(3H)pyrimidinone isolated in a lowtemperature argon matrix. The agreement between the spectrum simulated at the MP2 level and the experimental spectrum was remarkable and enabled positive assignment of the photoproduct structure. Photoreactions of matrixisolated 4(3H)pyrimidinones not methylated at the N3 nitrogen atom have also been studied. For these compounds, three types of photoreactions were observed, i.e., phototautomerism, ring opening, and Dewar structure formation. The relative probabilities of the three competing reaction directions and their dependencies on ring substitutions have been investigated, and results are presented.
 Mordzinski, A., Kownacki, K., Les, A., Oyler, N. A., Adamowicz, L., Langkilde, F. W., & Wilbrandt, R. (1994). Protontransferring systems studied by vibrational spectroscopy and theoretical ab initio calculations. The S_{0} and T_{1} states of [2,2′bipyridine]3,3′diol. Journal of physical chemistry, 98(20), 52125220.More infoAbstract: Tautomerization and phototautomerization of [2,2′bipyridine]3,3′diol is investigated by means of infrared, Raman, and timeresolved resonance Raman spectroscopies and ab initio theoretical calculations. Full ab initio SCF (selfconsistent field method) geometry optimization followed by MBPT (2)/631G** (secondorder manybody perturbation theory) energy calculations of both tautomeric forms in the ground state and the lowest excited triplet state is reported and discussed. Two tautomeric forms. A (dihydroxy) and B (dioxo) are found to be minima on the S0 and T1 potential energy surfaces. While form A is the most stable one in the ground state, the energetic ordering is reversed when going to the triplet state. A good agreement between the experimental and calculated vibrational spectra allows a reliable assignment of most vibrational bands corresponding to tautomer A in the ground state. To assign the observed triplettriplet optical transition and estimate intensities of resonance Raman bands in the triplet state, excitation energies and oscillator strengths of higher triplet states relative to T1 are calculated. Tentative assignments for the resonance Raman bands observed in the lowest excited triplet state are given. All observed vibrational bands are assigned to totally symmetric modes of the most stable B form. From a comparison of the observed and calculated T1 resonance Raman spectra, it is concluded that two transitions, namely, T1 → T5 and T1 → T8, contribute to the resonance Raman scattering and to the triplettriplet absorption band observed around 430 nm. These states are both of Ag symmetry and are calculated at 3.15 (T5) and 4.80 eV (T8) above T1 (Bu). In view of the fact that the exciting wavelength (around 430 nm, 2.88 eV) is substantially out of resonance with the calculated T1 → T8 transition, vibronic coupling between T5 and T8 is considered as a possibility.
 Oyler, N. A., & Adamowicz, L. (1994). Theoretical ab initio calculations of the electron affinity of thymine. Chemical Physics Letters, 219(34), 223227.More infoAbstract: It was determined in ab initio calculations that an electron can attach to thymine, forming a stable anion. The electron is attached predominantly to the positive end of the molecular dipole moment; however, some contribution from the covalent character is also present. The adiabatic electron affinity for this process is estimated to be only 0.088 eV. © 1994.
 Piecuch, P., & Adamowicz, L. (1994). Solving the singlereference coupledcluster equations involving highly excited clusters in quasidegenerate situations. The Journal of Chemical Physics, 100(8), 58575869.More infoAbstract: A new procedure for improving the convergence of the reduced linear equation method for solving the singlereference (SR) coupledcluster (CC) equations in highly degenerate cases, where the convergence of the iterative process is often destabilized by "dangerous" denominators, is discussed and implemented. Emphasis is placed on the SR CC methods involving highly excited clusters, such as the CC method with singles, doubles, triples, and quadruples (CCSDTQ). The new algorithm is based on rearranging and quasilinearizing the nonlinear terms and correcting the energy denominators by the quasilinearized terms. The method is tested in the SR CCSDTQ calculations for a dissociating single bond. © 1994 American Institute of Physics.
 Piecuch, P., & Adamowicz, L. (1994). Stateselective multireference coupledcluster theory using multiconfiguration selfconsistentfield orbitals. A model study on H_{8}. Chemical Physics Letters, 221(12), 121128.More infoAbstract: The recently developed stateselective (SS) multireference (MR) coupledcluster (CC) method is applied to a prototype molecular system composed of eight hydrogen atoms. To obtain a better description of the zerothorder wavefunction in the quasidegenerate region, we employ orbitals resulting from the complex active space multiconfigurational selfconsistentfield (MC SCF) optimization. Comparison with the results obtained using the groundstate restricted HartreeFock SCF orbitals indicates invariant character of the SS CC theory with respect to the orbital choice, contrary to the behavior of the Hilbertspace stateuniversal MR CC method. © 1994.
 Piecuch, P., & Adamowicz, L. (1994). Stateselective multireference coupledcluster theory employing the singlereference formalism: Implementation and application to the H_{8} model system. The Journal of Chemical Physics, 100(8), 57925809.More infoAbstract: The new stateselective (SS) multireference (MR) coupledcluster (CC) method exploiting the singlereference (SR) particlehole formalism, which we have introduced in our recent paper [P. Piecuch, N. Oliphant, and L. Adamowicz, J. Chem. Phys. 99, 1875 (1993)], has been implemented and the results of the pilot calculations for the minimum basisset (MBS) model composed of eight hydrogen atoms in various geometrical arrangements are presented. This model enables a continuous transition between degenerate and nondegenerate regimes. Comparison is made with the results of SR CC calculations involving double (CCD), single and double (CCSD), single, double, and triple (CCSDT), and single, double, triple, and quadruple (CCSDTQ) excitations. Our SS CC energies are also compared with the results of the Hilbert space, stateuniversal (SU) MR CC(S)D calculations, as well as with the MR configuration interaction (CI) results (with and without Davidsontype corrections) and the exact correlation energies obtained using the full CI (FCI) method. Along with the groundstate energies, we also analyze the resulting wave functions by examining some selected cluster components. This analysis enables us to assess the quality of the resulting wave functions. Our SS CC theory truncated at double excitations, which emerges through selection of the most essential clusters appearing in the full SR CCSDTQ formalism [SS CCSD (TQ) method] provides equally good results in nondegenerate and quasidegenerate regions. The difference between the groundstate energy obtained with the SS CCSD(TQ) approach and the FCI energy does not exceed 1.1 mhartree over all the geometries considered. This value compares favorably with the maximum difference of 2.8 mhartree between the SU CCSD energies and the FCI energies obtained for the same range of geometries. The SS CCSD(T) method, emerging from the SR CCSDT theory through selection of the most essential clusters, is less stable, since it neglects very important semiinternal quadruple excitations. Unlike the genuine multideterminantal SU CC formalism, our SS CC approach is not affected by the intruder state problem and its convergence remains satisfactory in nondegenerate and quasidegenerate regimes. © 1994 American Institute of Physics.
 Roehrig, G. H., Oyler, N. A., & Adamowicz, L. (1994). Can electron attachment alter tautomeric equilibrium of guanine? Theoretical ab initio study. Chemical Physics Letters, 225(13), 265272.More infoAbstract: Ab initio calculations performed in this work show that the two major tautomers of guanine have positive, but very small, electron affinities. The adiabatic values are 0.034 and 0.00038 eV for the oxoamino and hydroxyamino forms, respectively. The large difference in the electron affinities should lead to a significant difference in the tautomeric equilibria of the neural and anionic guanines in the gas phase. © 1994.
 SaintMartin, H., OrtegaBlake, I., Leś, A., & Adamowicz, L. (1994). The role of hydration in the hydrolysis of pyrophosphate. A Monte Carlo simulation with polarizabletype interaction potentials. Biochimica et Biophysica Acta (BBA)/Protein Structure and Molecular, 1207(1), 1223.More infoPMID: 8043601;Abstract: The exchange of energy in biochemical reactions involves, in a majority of cases, the hydrolysis of phosphoanhydrides (POP). This discovery has lead to a long discussion about the origin of the high energy of such bonds, and to a proposal that hydration plays a major role in the energetics of the hydrolysis. This idea was supported by recent ab initio quantum mechanical calculations (SaintMartin et al. (1991) Biochim. Biophys. Acta 1080, 205214) that predicted the hydrolysis of pyrophosphate is exothermic in the gas phase. This exothermicity can account for only a half of the total energy release that one measures in aqueous solutions. Here we address the problem of hydration of the reactants and products of the pyrophosphate hydrolysis by means of Monte Carlo simulations, employing polarizable potentials whose parameters are fitted to energy surfaces computed at the SCF/631G** level of the theory. The present results show that the hydration enthalpies of the reactants and products contribute significantly to the total energy output of the pyrophosphate hydrolysis. The study predicts that both, the orthophosphate and the pyrophosphate, have hydration spheres with the water molecules acting as proton acceptors in the POHḑotO(water) hydrogen bonds. These water molecules weakly repel the water molecules in the further hydration spheres. The perturbation of the structure of the solvent caused by the presence of the solute molecules is short ranged: after ca. 5Å from the P atoms, the energy and the structure of water correspond to bulk water. Due mainly to nonadditive effects, the molecular structure of the hydrated pyrophosphate is quite different from two fused structures of the hydrated orthophosphates. The hydration sphere of pyrophosphate is very loose and has a limited effect on the water network, whereas for orthophosphate it has a well developed shell structure. Hence, upon hydration there will be both a gain in hydration enthalpy and a gain in entropy because of distortion of the water molecular network. © 1994.
 Slanina, Z., Lee, S. ., François, J., Kurtz, J., & Adamowicz, L. (1994). Inversion of the C_{8} nonplanar ring. Chemical Physics Letters, 223(56), 397401.More infoAbstract: Inversion of the recently reported nonplanar cyclic minimumenergy structure of C8 (D2d symmetry) is studied by means of the secondorder MøllerPlesset perturbation treatment with the 631G* basis set. Two new planar cyclic saddle points have been found, possessing D4h and C2h symmetry and exhibiting two and one imaginary frequencies, respectively. The former and the latter saddle point are located 71 and 54 kJ/mol, respectively, above the D2d minumum. Calculated harmonic vibrational frequencies for the activated complex C2h are presented. © 1994.
 Slanina, Z., Lee, S., Uhlík, F., & Adamowicz, L. (1994). AM1 computed relative equilibrium populations of the ( 5 6) and ( 5 6) methanobuckminsterfullerenes C_{61}H_{2}. Chemical Physics Letters, 228(45), 490494.More infoAbstract: Bridging by CH2 across the two different CC bonds in C60 is treated by the AM1 method. The C2v ( 6 6) isomer is located 29 kJ/mol above the Cs ( 5 6) structure. The computed equilibrium ratios of the isomers agree reasonably well with the available observations. A fit to experiment (based on the AM1 partition functions) yields for the isomer separation 7.6 kJ/mol, this being the best estimation at present. © 1994.
 Slanina, Z., Leś, A., & Adamowicz, L. (1994). Complexes of 4aminopyrimidine and 4hydroxypyrimidine with water: computed relative thermodynamic stabilities. Thermochimica Acta, 231(C), 6168.More infoAbstract: Relative thermodynamic stabilities in two series of isomeric complexes of substituted pyrimidine (4aminopyrimidine; 4hydroxypyrimidine, 4oxypyrimidine) with water have been computed. Description of the isomers is based on ab initio SCF calculation, with energetics corrected by the second order perturbation treatment. The relative stabilities are evaluated in terms of partition functions constructed from the calculated dimeric parameters. In each of the sets several relative stability interchanges are found. The interchanges are manifested in a considerable heat capacity term enhancement. © 1994.
 Slanina, Z., Uhlik, F., Lee, S., & Adamowicz, L. (1994). AM1 Computations of C_{60}O_{2}. Fullerene Science and Technology, 2(1), 7388.More infoAbstract: The C60O2 structures with a trans ( the most distant) arrangement of the oxygen atoms are treated by means of the AM1 quantumchemical method. Full geometry optimization of the D2h and C2h structures is performed together with vibrational harmonic analysis (the latter structures is lower in energy by about 103 kJ/mol). Lowlying triplet states are pointed out. In the D2h structure the Obridged CC bond is elongated to 1.55 A, while in the C2h is broken.
 Slanina, Z., Uhlik, F., Lee, S., & Adamowicz, L. (1994). C_{61}H_{2} fulleroid: AM1 computational study. Fullerene Science and Technology, 2(1), 1324.More infoAbstract: The C61H2 fulleroid is treated by means of the AM1 quantumchemical method. Full geometry optimization of its C2v structure is performed together with vibrational harmonic analysis. The computed length of the CC bond between two cagehexagons bridged by the tetrahedral C atom is 1.579 A. Vibrational frequencies and their IR intensities are reported.
 Smets, J., Adamowicz, L., & Maes, G. (1994). Combined matrixisolation FTIR and ab initio 631++G** study of Hbonded complexes between water and molecules modelling cytosine or isocytosine. Journal of Molecular Structure, 322(C), 113130.More infoAbstract: A stepbystep experimental and ab initio SCF/631++G** procedure is described for the interpretation of the matrix FTIR spectra of Hbonded complexes of cytosine or isocytosine with water. The method involves a detailed study of several compounds, each modelling one or a few cytosine sites and/or tautomers in order of increasing complexity. Results are presented for tautomerization processes in 2OHpyridine, 4OHpyrimidine and isocytosine, and for Hbonded complexes of water with pyrimidine, 4OHpyridine and 4NH2pyridine. The whole series of obtained results has allowed the setting up of a largescale vibration correlation diagram for water complexed with N and Obases. It also allows some new, important conclusions to be drawn about the major problem of scaling ab initio predicted frequencies for anharmonic protondonor vibrational modes. © 1994.
 Vranken, H., Smets, J., Maes, G., Lapinski, L., Nowak, M. J., & Adamowicz, L. (1994). Infrared spectra and tautomerism of isocytosine; an ab initio and matrix isolation study. Spectrochimica Acta Part A: Molecular Spectroscopy, 50(5), 875889.More infoAbstract: Prototropic tautomerism of isocytosine has been investigated using both theoretical ab initio and experimental matrix isolation IR methods. The coexistence of the aminohydroxy and aminooxo N(3)H forms, with a clear predominance of the hydroxy form, was observed. The tautomerization constant [oxo]/[hydroxy] obtained from experimental and calculated IR intensities was 0.11 at the microoven temperature of 400 K. The ab initio prediction of the relative energies of the tautomers is in reasonable agreement with the experimental estimation. The change of the tautomeric form oxo→hydroxy upon UV irradiation was used to separate the IR spectra of both tautomers. A theoretically assisted interpretation of the IR spectra of both observed tautomers is proposed. © 1994.
 Kozlowski, P. M., & Adamowicz, L. (1993). Equivalent quantum approach to nuclei and electrons in molecules. Chemical Reviews, 93(6), 20072022.
 Kozlowski, P. M., & Adamowicz, L. (1993). Nonadiabatic variational calculations for the ground state of the positronium molecule. Physical Review A, 48(3), 19031908.More infoAbstract: For a fourparticle system consisting of two electrons and two positrons, the nonadiabatic wave function is constructed with the use of an expansion in terms of explicitly correlated Gaussiantype basis functions and a Cartesiancoordinate laboratory frame. Motions of all particles are correlated at the same time in the wave function. The energy of the centerofmass motion is effectively eliminated from the total nonrelativistic energy of the system by defining the variational principle based on the internal Hamiltonian. The groundstate energy is computed for different lengths of Gaussian expansions and values are compared with previous literature results. Our best estimation of the binding energy of the positronium molecule is 0.435 eV. © 1993 The American Physical Society.
 Les, A., Adamowicz, L., & Rode, W. (1993). Mechanism of thymidylate synthase inhibition by N^{4}hydroxy(N^{4}hydroxy 5fluoro)dCMP in view of the structure and conformation of N^{4}hydroxy (N^{4}hydroxy5fluoro)cytosine calculated by the AB initio quantum mechanical methods. Advances in Experimental Medicine and Biology, 338, 621624.More infoPMID: 8304194;
 Leś, A., Adamowicz, L., & Rode, W. (1993). Structure and conformation of N^{4}hydroxycytosine and N^{4}hydroxy5fluorocytosine. A theoretical ab initio study. BBA  Gene Structure and Expression, 1173(1), 3948.More infoPMID: 8485152;Abstract: Optimal molecular geometries and molecular energies were obtained for N4hydroxycytosine and its 5fluoro congener with the use of the theoretical ab initio quantum mechanical calculations within the Self Consistent Field method corrected for the electron correlation effects by the secondorder Many Body Perturbation Theory (SCF + MBPT(2)). The 631G Gaussian basis set was employed. Several tautomeric and rotameric forms were considered. For N4hydroxycytosine and N4hydroxy5fluorocytosine the imino tautomer (in the conformation syn relatively to the N3nitrogen atom) appeared to be the most stable form. The imino tautomer of N4hydroxycytosine in the anti rotameric form is by 12.8 kJ mol1 less stable than the iminosyn form. The 5fluoro substituent raises the energy difference between the syn and anti rotamers up to 38.5 kJ mol1. The potential energy barrier for the synanti rotation in the imino form of N4hydroxycytosine is estimated to be about 180 kJ/mol. The results presented in this paper suggest that the synimino and antiimino forms can be treated as two structural isomers that do not interconvert at temperatures relevant to biochemical conditions. The theoretical results also show that the amino tautomeric forms do not compete with the imino forms in the gasphase and in nonpolar and weaklypolar environment. In a polar environment (e.g., in aqueous solutions), however, one may expect an increased population of the amino forms. Qualitatively, the results of the present study agree well with the available experimental and theoretical data for N4hydroxycytosine and some of its derivatives. The implications of the present study are discussed in relation to the molecular mechanisms of mutagenesis caused by NH2OH and of enzyme (thymidylate synthase) inhibition by N4hydroxydeoxycytidine monophosphate. © 1993.
 McCarthy, W. J., Roehrig, M. A., Chen, Q., Henderson, G. L., Adamowicz, L., & Kukolich, S. G. (1993). Microwave measurements and ab initio dynamics of the large amplitude ring puckering motion in 2sulpholene. The Journal of Chemical Physics, 99(10), 73057314.More infoAbstract: Microwave measurements were made on the rotational spectrum of 2sulpholene using a modified FlygareBalle pulsed beam Fourier transform spectrometer. Analysis and calculations provided information on the large amplitude ring puckering vibration of this system. Twelve and six rotational transitions were measured for the v=0 and v=1 states of the ring puckering vibration, respectively. The transitions for each vibrational state were fitted to a Watson's A reduced Hamiltonian including terms for quartic distortion yielding for v=0 the values B=2125.96(6), C=983.28(8), ΔJK=0.664(4), ΔK=0.34(4) MHz, and for v=1 the values A=3995(26), B=2128.3(1), C=1984.6(1), ΔJK=0.8(1), ΔK= 32(6) MHz. Subsequently, ab initio calculations were performed at the selfconsistentfield (SCF)/321G*, MP2/ 631G *, and MP4/631G* levels of theory to determine the barrier to inversion. The MP4/631G* barrier was ΔE=116 cm1, and can be considered to be the most accurate barrier value calculated in this study. An ab initio potential energy curve was calculated at the SCF/321G* level in terms of a single parameter (ω) describing the large amplitude motion of the ring puckering. Vibrationcoordinate dependence of the effective reduced mass associated with this large amplitude motion and the resultant kinetic energy expression was determined. The solutions of a onedimensional Schrödinger equation solved within this double well potential yield a separation between the v=0 and v=1 large amplitude motion vibrational states of 8 cm1 when the effective reduced mass was assumed constant, and a separation of 9 cm 1 when the effective reduced mass was expressed as a function of the ω coordinate. The v=0 and v=1 eigenfunctions for the SCF ring puckering potential were found to give vibrationally averaged rotational constants in good agreement with those obtained from the microwave spectrum. © 1993 American Institute of Physics.
 Oliphant, N., & Adamowicz, L. (1993). Multireference coupled cluster method for electronic structure of molecules. International Reviews in Physical Chemistry, 12(2), 339362.More infoAbstract: In this review we present a systematic derivation of the multireference coupled cluster theory based on the single reference formalism. The coupled cluster theories have recently emerged as one of the major method development activities in the electronic structure theory of atoms and molecules. Due to its sizeextensive nature, using the coupled cluster method the total electronic energy of the system can be determined with the same relative accuracy as the total electronic energies of the fragments which the system separates into in the process of chemical decomposition. This feature is essential for the correct theoretical determination of dissociation energies as well as other molecular properties. One of the most difficult challenges in advancing the coupled cluster theory has been the development of the multireference coupled cluster methodology, i.e. generating a scheme which allows the reference function to incorporate more than one Slater determinant. Such development would enable a very accurate ab initio treatment of general categories of openshell systems as well as the treatment of systems with stretched multiple bonds leading to a more precise determination of vibrational spectra. In this article we review our recent results in the development of a multireference coupled cluster theory. The reader will be first acquainted with the second quantization formalism, then guided through the derivation of the single reference coupled cluster theory, and finally presented with the multireference formalism. We have included several numerical examples illustrating the performance of the single reference and multireference coupled cluster methods.
 Oyler, N. A., & Adamowicz, L. (1993). Electron attachment to uracil. Theoretical ab initio study. Journal of Physical Chemistry, 97(42), 1112211123.More infoAbstract: Ab initio calculations indicate that the uracil molecule can bind an electron and form a stable dipolebound anion. The adiabatic electron affinity of this process is estimated to be only 0.003 15 hartree (0.086 eV). © 1993 American Chemical Society.
 Piecuch, P., Oliphant, N., & Adamowicz, L. (1993). A stateselective multireference coupledcluster theory employing the singlereference formalism. Journal of Chemical Physics, 99(3), 18751900.More infoAbstract: A new stateselective multireference (MR) coupledcluster (CC) method exploiting the singlereference (SR) particlehole formalism is described. It is an extension of a simple tworeference formalism, which we presented in our earlier paper [N. Oliphant and L. Adamowicz, J. Chem. Phys. 94, 1229 (1991)], and a rigorous formulation of another method of ours, which we obtained as an approximation of the SRCC approach truncated at triple excitations (SRCCSDT) [N. Oliphant and L. Adamowicz, J. Chem. Phys. 96, 3739 (1992)]. The size extensivity of the resulting correlation energies is achieved by employing a SRCClike ansatz for the multideterminantal wave function. General considerations are supplemented by suggesting a hierarchy of approximate schemes, with the MRCCSD approach (MRCC approach truncated at double excitations from the reference determinants) representing the most important one. Our stateselective MRCCSD theory emerges through a suitable selection of the most essential cluster components appearing in the full SRCCSDTQ method (SRCC method truncated at quadruple excitations), when the latter is applied to quasidegenerate states. The complete set of equations describing our MRCCSD formalism is presented and the possibility of the recursive intermediate factorization [S. A. Kucharski and R. J. Bartlett, Theor. Chim. Acta 80, 387 (1991)] of our approach, leading to an efficient computer algorithm, is discussed. © 1993 American Institute of Physics.
 Rostkowska, H., Nowak, M. J., Lapinski, L., Bretner, M., Kulikowski, T., Les, A., & Adamowicz, L. (1993). Theoretical and matrixisolation experimental studies on 2thiocytosine and 5fluoro2thiocytosine. BBA  Gene Structure and Expression, 1172(3), 239246.More infoPMID: 8448202;Abstract: 2Thiocytosine (s2Cyt) and 5fluoro2thiocytosine (f5s2Cyt) were studied by means of IR spectroscopy under different environmental conditions: isolated in lowtemperature inert gas matrices, associated in thin amorphous and polycrystalline films. The compounds isolated in matrices were only very slightly influenced by the environment. From the analysis of the IR spectra of both compounds it appears that they exist in inert gas matrices only in the aminothiol tautomeric form. Strong environmental effects were observed for s2Cyt and f5s2Cyt deposited in the form of thin polycrystalline films. Contrary to matrices, in polycrystalline films the aminothione form dominates for both s2Cyt and f5s2Cyt. The experimental findings are in agreement with the ab initio quantum mechanical calculations of the relative total energies of the tautomeric forms. Those energies were calculated using the Self Consistent Field method corrected for electron correlation effects with the use of the secondorder manybody perturbation theory (SCF + MBPT(2)). The theoretical calculations show that the aminothiol tautomeric form is more stable than the aminothione form by 38 kJ mol1 and 48 kJ mol1 for s2Cyt and f5s2Cyt, respectively. Both molecules, s2Cyt and f5s2Cyt, may also appear in the uracillike iminothione tautomeric form, which is predicted to be only 8 kJ mol1 less stable than the aminothione form. A new method of the preparation of f5s2Cyt is reported. © 1993.
 Rostkowska, H., Nowak, M., Lapinski, L., Bretner, M., Kulikowski, T., Leś, A., & Adamowicz, L. (1993). Infrared spectra of 2thiocytosine and 5fluoro2thiocytosine; experimental and ab initio studies. Spectrochimica Acta Part A: Molecular Spectroscopy, 49(4), 551565.More infoAbstract: IR absorption spectra of 2thiocytosine and 5fluoro2thiocytosine isolated in lowtemperature, inert gas matrices and in thin polycrystalline films are reported. The spectra of aminothiol tautomeric forms of the compounds were studied for matrix isolated species, whereas the spectra of aminothione tautomeric forms were observed for polycrystalline films. The experimental spectra were compared with the spectra predicted with the ab initio SCF/631G** method. The observed IR absorption bands in the spectra of monomers (aminothiol forms) were assigned to the theoretically calculated normal modes. © 1993.
 Slanina, Z., & Adamowicz, L. (1993). A computational study of the ozone dimer. Journal of Atmospheric Chemistry, 16(1), 4146.More infoAbstract: The dimer of ozone is treated ab initio by the secondorder MollerPlesset perturbation approach with the 631G* and 631+G* basis sets (with an evaluation of the basis set superposition error and the fourthorder corrections). It is found that the minimumenergy structure exhibits Cs symmetry (with some patterns resembling the structure of the water dimer). The calculated dimerization energy varies between 13 and 1 kJ/mol. Monomerdimer shifts in the vibrational frequencies are rather small (about 10 cm1 or less) while the intermolecular frequencies vary between 30 and 120 cm1. The ozone dimer could influence some spectral observations under atmospheric conditions. from Authors
 Slanina, Z., & Adamowicz, L. (1993). MNDO study of charged complexes of dodecahedronshaped C_{20} with Li. Journal of Molecular Structure: THEOCHEM, 281(1), 3337.More infoAbstract: Singly charged complexes of dodecadedronshaped C20 with Li were studied by means of the MNDO semiempirical method and optimal structures, energies and harmonic vibrations were obtained for C20, C202, [C20·Li]+ and [C20·Li]. Two locations of the Li atom were treated (external and internal) and the external complexes were found to be thermodynamically more stable by at least 600 kJ mol1. The internal location is, however, kinetically stabilized by a potential barrier of about 1000 kJ mol1. The interactions with the Li atom do not produce substantial changes in the cage geometry, but they can be well seen in the vibrational patterns. © 1993.
 Slanina, Z., & Adamowicz, L. (1993). Two maxima dependencies of isomerism contribution to heat capacity of N_{2}OHCN & COC_{2}H_{2} systems in low temperature region. Zeitschrift für Physik D Atoms, Molecules and Clusters, 26(1 Supplement), 338340.More infoAbstract: Temperature effects on heat capacity in four and three component systems N2OHCN and COC2H2, respectively, were computed. Structural and vibrational parameters originated from the second order MøllerPlesset perturbation treatment with the 631++G** basis set while relative energies were of a still advanced nature. Isomerism contribution to heat capacity exhibits two temperature maxima at 43 and 205 or 15 and 83 K in N2OHCN or COC2H2. The highest reported isomeric enhancement of the total heat capacity is about 27 %. © 1993 SpringerVerlag.
 Slanina, Z., Leś, A., & Adamowicz, L. (1993). Complexes of 2aminopyrimidine, 2hydroxypyrimidine and their tautomers with water. Relative thermodynamic stabilities. Thermochimica Acta, 228(C), 17.More infoAbstract: Relative stabilities in two pairs of complexes of tautomers of substituted pyrimidine (2aminopyrimidine and 2iminopyrimidine; 2hydroxypyrimidine and 2oxypyrimidine) with water have been computed. Description of the isomers is based on ab initio SCF calculation, with energetics corrected by the secondorder perturbation treatment. The relative stabilities are evaluated in terms of partition functions constructed from the calculated dimeric parameters. Complexes of 2hydroxypyrimidine and 2oxypyrimidine with water were found to coexist at higher temperatures. However, the overall heat capacity can be enhanced by about 3%, at most, by the isomeric interplay. The relation to nonequilibrium situations is discussed. © 1993.
 Slanina, Z., Leś, A., & Adamowicz, L. (1993). The effects of temperature on the relative stabilities of five complexes of 4hydroxypyridine and water. Thermochimica Acta, 224(C), 1318.More infoAbstract: The relative stabilities of five isomeric complexes of 4hydroxypyridine with water have been computed. Description of the isomers is based on ab initio SCF calculation, with energetics corrected by the secondorder perturbation treatment. The relative stabilities are evaluated in terms of partition functions constructed from the calculated dimeric parameters. A possibility of relative stability interchange is pointed out in the higher temperature region. However, the overall heat capacity of the system can be enhanced by the isomeric interplay by 7%, at most. © 1993.
 Slanina, Z., Martin, J. M., François, J., & Adamowicz, L. (1993). Why computed entropies of quasilinear species are sometimes random?. Journal of Molecular Structure: THEOCHEM, 280(1), 8387.More infoAbstract: Using the example of a gaussian 90 computation of the linear BBNB molecule in its singlet state, it is shown that the calculated entropy value can be considerably different from that deduced from seemingly equivalent computations performed on a quasilinear species. This problem is interpreted in terms of the limiting behaviour of the conventional rotational partition function of a polyatomic molecule. The problems originate from a routine application of quantumchemical programs to such linear cases. The results are shown to be important for linear systems optimized (owing to, for example, better convergence properties) as quasilinear systems. The finding explains why some published computed entropies of essentially linear species cannot be reproduced. © 1993.
 Slanina, Z., Uhlík, F., & Adamowicz, L. (1993). A classification of 200 isomerizations among 51 isotopomers of C_{8} (D_{2d}). Journal of Radioanalytical and Nuclear Chemistry Articles, 170(2), 373380.More infoAbstract: The set of all 51 possible12C/13C isotopomers of a C8 cluster of D2d symmetry has been studied by means of the harmonic vibration analysis based on the force field resulting from the second order MøllerPlesset (MP2) perturbation treatment with the 631G* basis set. The isotopomers can undergo 200 isomerizations which can be classified into 5 distinct groups using the properties of sums of the squares of harmonic vibrational frequencies. © 1993 Akadémiai Kiadó.
 Slanina, Z., Uhlík, F., & Adamowicz, L. (1993). Classification of 486 isomerizations among 72^{12}C/^{13}C isotopomers of cyclic C_{7}. Journal of Radioanalytical and Nuclear Chemistry Articles, 170(1), 107116.More infoAbstract: Isotopic rules are studied for calculated vibrational frequencies of all possible 7212C/13C isotopomers of the C7 cluster of the C2v symmetry. The isotopomers can undergo 486 isomerizations which are decomposed into 27 subclasses using properties of sums of the squares of the vibrational frequencies. © 1993 Akadémiai Kiadó.
 Adamowicz, L. (1992). C_{2}H_{2}CO complex. Ab initio calculations with the coupledcluster method and firstorder correlation orbitals. Chemical Physics Letters, 192(23), 199204.More infoAbstract: Three equilibrium conformers are identified for the C2H2CO complex. Coupledcluster calculations with the firstorder correlation orbitals indicate that the two linear structures, i.e. carbon and oxygenbonded, are isoenergetic and should coexist in the gas phase. However, due to smaller internal flexibility, the carbonbonded complex should have a more definite structure. © 1992.
 Adamowicz, L. (1992). NNOHCN complex. Ab initio calculations with the coupled cluster method and firstorder correlation orbitals. Chemical Physics, 165(23), 281286.More infoAbstract: Four equilibrium conformers are identified for the NNOHCN complex. Coupled cluster calculations with the firstorder correlation orbitals indicate that the oxygenhydrogenbonded linear structure has the lowest energy, by 0.26 kcal/mol lower, than the experimentally characterized "slipped parallel" structure. The result lends support to suggestions that there should be at least two stable isomers of NNOHCN in the gas phase. © 1992.
 Adamowicz, L. (1992). Stable and metastable states of the Si^{}_{5} anion. Theoretical study. Chemical Physics Letters, 188(12), 131134.More infoAbstract: The Si5 cluster forms a bitrigonal pyramid. This structure can form some stable and metastable (autodetaching) states with an additional electron, which attaches to the cluster surface. Energies of some of those states are determined based on ab initio calculations with the use of the coupledcluster method combined with the firstorder correlation orbital method. It is found that at least four states, i.e. X2A″2, 2E′, 2E″ and (2)2E′, lie below the electron ejection threshold. © 1992.
 Kozlowski, P. M., & Adamowicz, L. (1992). Implementation of analytical first derivatives for evaluation of the manybody nonadiabatic wave function with explicitly correlated Gaussian functions. The Journal of Chemical Physics, 96(12), 90139024.More infoAbstract: A nonadiabatic manyparticle wave function is generated using an expansion in terms of explicitly correlated Gaussiantype basis functions. In this approach, motions of all particles are correlated at the same time, and electrons and nuclei are distinguished via permutational symmetry. We utilize our newly proposed nonadiabatic variational approach [P. M. Kozlowski and L. Adamowicz, J. Chem. Phys. 95, 6681 (1991)], which does not require the separation of the internal and external motions. The analytical first derivative of the variational functional with respect to the nonlinear parameters appearing in the basis functions are derived and implemented to find the minimum. Numerical examples for the ground state of the hydrogen molecule are presented. © 1992 American Institute of Physics.
 Kozlowski, P. M., & Adamowicz, L. (1992). NewtonRaphson optimization of the manybody nonadiabatic wave function expressed in terms of explicitly correlated Gaussian functions. The Journal of Chemical Physics, 97(7), 50635073.More infoAbstract: A nonadiabatic manybody wave function is represented in terms of explicitly correlated Gaussiantype basis functions. Motions of all particles (nuclei and electrons) are treated equally and particles are distinguished via permutational symmetry. The nonadiabatic wave function is determined in a variational calculation with the use of the method proposed recently [P. M. Kozlowski and L. Adamowicz, J. Chem. Phys. 95, 6681 (1991)]. In this approach no direct separation of the centerofmass motion from the internal motion is required. The theory of analytical first and second derivatives of the variational functional with respect to the Gaussian exponents and its computational implementation in conjunction with the NewtonRaphson optimization technique is described. Finally, some numerical examples are shown. © 1992 American Institute of Physics.
 Lapinski, L., Nowak, M. J., Fuiara, J., Leś, A., & Adamowicz, L. (1992). Relation between structure and tautomerism in diazinones and diazinethiones. An experimental matrix isolation and theoretical ab initio study. Journal of Physical Chemistry, 96(15), 62506254.More infoAbstract: Tautomerism of 2pyridinone, 2pyrimidinone, 2pyrazinone, 4pyrimidinone, 3pyridazinone, and their thio analogs isolated in lowtemperature inert gas matrices has been investigated by means of IR spectroscopy. The UVinduced phototautomeric reaction has been used to separate the IR spectra of the tautomers. It is found that the relative stability of the oxo and hydroxy tautomers of the above compounds depends in a systematic way on the relative position of the lactam group and the second nitrogen atom in the ring. In the analogous compounds with an exocyclic sulfur atom, the stability of the thiol forms (with respect to the corresponding thione forms) is considerably higher than the stability of the hydroxy forms (with respect to the corresponding oxo forms) in the compounds with exocyclic oxygen. The trend, which emerges from the experimental observations, is correctly reproduced by the theoretical ab initio calculations performed at the SCF + MBPT(2) level with the DZP basis set. © 1992 American Chemical Society.
 Leś, A., Adamowicz, L., Nowak, M. J., & Lapinski, L. (1992). The infrared spectra of matrix isolated uracil and thymine: An assignment based on new theoretical calculations. Spectrochimica Acta Part A: Molecular Spectroscopy, 48(10), 13851395.More infoAbstract: The IR spectra of uracil and thymine predicted theoretically at the ab initio HartreeFock level with 631G basis set are reported and compared with Ar matrix experimental spectra. The IR spectra computed at the SCF/631G level reproduce the experimental spectra with an accuracy which allows a reliable vibrational assignment. A split valence basis set, augmented with polarization functions on all atoms, was found to be quite sufficient for a reliable prediction and assignment of the IR absorption bands in the spectra of mediumsized molecules. © 1992.
 Leś, A., Adamowicz, L., Nowak, M. J., & Lapinski, L. (1992). Theoretical interpretation of the gas phase equilibrium of 2hydroxypyridine/2(1H)pyridinone. Journal of Molecular Structure: THEOCHEM, 277(C), 313327.More infoAbstract: Theoretical ab initio calculations are performed to provide support for our hypothesis that the gas phase ratio of the 2hydroxypyridine/2(1H)pyridinone tautomers results from the thermodynamic equilibrium which exists between cyclic hydrogenbonded oxooxo and hydroxyhydroxy dimers. We therefore, disprove the monomolecular mechanism of the tautomerization process, which has frequently been assumed in the past to estimate the free energy difference between the monomeric tautomers based on the experimentally determined tautomer ratio. © 1992.
 Millam, J. M., & Adamowicz, L. (1992). Numerical HartreeFock characterization of the metastable u4 state of the Ne2 ion. Physical Review A, 46(5), 23252329.More infoAbstract: The metastable u4 state of the Ne2 anion is examined with the use of the numerical HartreeFock method. The results suggest that due to its similarity to the experimentally discovered g4 state of He2, this state should also be long lived. © 1992 The American Physical Society.
 Nowak, M. J., Lapinski, L., Fulara, J., Les, A., & Adamowicz, L. (1992). Matrix isolation IR spectroscopy of tautomeric systems and its theoretical interpretation. 2Hydroxypyridine/2(1H )pyridinone. Journal of Physical Chemistry, 96(4), 15621569.More infoAbstract: Prototropic tautomerism of a model system of 2hydroxypyridine/2(1H)pyridinone has been investigated in lowtemperature inert matrices and in the gas phase. The ratio of tautomer concentrations, [hydroxy]:[oxo], for the 2pyridinone sample deposited in Ar or N2 matrices was estimated as 2.80:1 and 2.99:1, respectively. A similar ratio of tautomers was observed in the gas phase. The matrix and gasphase infrared spectra of 2hydroxypyridine and 2(1H)pyridinone are reported. The identification of the bands corresponding to the two tautomers was accomplished by performing the UVinduced transformation of the oxo tautomer into the hydroxy tautomer in matrices, and with the use of theoretically predicted vibrational spectra. Ab initio theoretical calculations of the infrared spectra were performed within the harmonic approximation at the SCF/631G** level. A good agreement between experimental and theoretically predicted frequencies and intensities enabled a reliable assignment of IR bands in the spectra of both tautomers. © 1992 American Chemical Society.
 Oliphant, N., & Adamowicz, L. (1992). Converging the singlereference coupledcluster equations. Chemical Physics Letters, 190(12), 1318.More infoAbstract: A scheme for stabilizing the reduced linear equation method of solving the singlereference coupled system of equations is developed and implemented. When one or more secondary determinants become as important as the primary determinant in describing the chemical system, numerical instabilities frequently arise in the procedure of solving the coupledcluster equations. This is avoided by "quasilinearizing" certain nonlinear terms in the equations and including them with the linear terms. A study on the LiH molecule with an internuclear separation of 9.045 au is used to demonstrate the effectiveness of this procedure. © 1992.
 Oliphant, N., & Adamowicz, L. (1992). The implementation of the multireference coupledcluster method based on the singlereference formalism. The Journal of Chemical Physics, 96(5), 37393744.More infoAbstract: A generalized version of the multireference coupledcluster method using a singlereference formalism, which we presented in an earlier paper, has been implemented. Any number of determinants, that differ from the formal reference determinant by single or double excitations, can now be included in the reference space. In the present implementation, the single and double excitations from the secondary reference determinants have been truncated to include only those that correspond to triple excitations from the formal reference determinant. Calculations are done on a few model systems, LiH, BH, and H2O, at equilibrium and stretched geometries. Comparisons are made with full configuration interaction (CI) treatment for the single bond stretch in LiH and BH, and the results are quite promising. For the water molecule, comparisons are made with the results obtained with the coupled cluster method truncated at triple excitations (CCSDT), as well as with the full CI results. While the multireference method did not do as well for the simultaneous twobond stretch in H2O as it did for the single bond cases, it did at least as well as the CCSDT at representing the points on the full CI potential curve. © 1992 American Institute of Physics.
 Slanina, Z., & Adamowicz, L. (1992). N_{2}OHCN and COC_{2}H_{2}: two associating systems with remarkable lowtemperature relativestability interchanges. Journal of Molecular Structure: THEOCHEM, 276(C), 133140.More infoAbstract: Relative stabilities of four and three minimumenergy structures in the N2O HCN and COC2H2 systems, respectively, were evaluated in terms of partition functions. Structural and vibrational parameters originated from the secondorder MøllerPlesset perturbation treatment with the 631 + + G** basis set. Relative energies were further refined within the coupled cluster method and firstorder correlation orbitals. Three relative stability interchanges were revealed in each system. The lowest interchange points appear at temperatures of 114 K and 13 K in the N2O HCN and COC2H2 systems, respectively. © 1992.
 Slanina, Z., & Adamowicz, L. (1992). On the relative stabilities of dodecahedronshaped and bowlshaped structures of C_{20}. Thermochimica Acta, 205(C), 299306.More infoAbstract: Purely carbonaceous aggregates, C20, were studied by the AM1 quantumchemical method. In addition to one dodecahedronshaped structure possessing C1 symmetry, another threedimensional species was revealed, namely a bowlshaped structure of C5v symmmetry. The temperature dependence of the relative stabilities of both structures is evaluated, showing that the fullerenic species prevails in the temperature region studied. However, at very high temperatures, a relativestability interchange is predicted. In view of the absence of any symmetry in the dodecahedronshaped structure, the species is treated both as an equimolar mixture of two enantiomers and as one pure optical form. The effects of the enantiomerism on the overall properties of C20 are briefly discussed. © 1992.
 Slanina, Z., Adamowicz, L., Bakowies, D., & Thiel, W. (1992). Fullerene C_{50} isomers: Temperatureinduced interchange of relative stabilities. Thermochimica Acta, 202(C), 249254.More infoAbstract: The species C50 is treated as a system composed of three local energy minima (D5h, D3, and C2v) found in recent modified neglect of diatomic overlap (MNDO) calculations. Although the D5h species corresponds to the deepest minimum it is the most stable structure only up to about 1390 K. Beyond this temperature the D3 species becomes relatively more populous. © 1992.
 Slanina, Z., Kurtz, J., & Adamowicz, L. (1992). A computational response to the C_{7} experimental challenge: lowenergy C_{2v} isomer. Chemical Physics Letters, 196(12), 208212.More infoAbstract: Recent experimental evidences of C7 cluster isomerism are supported by the secondorder MølerPlesset (MP2) perturbation treatment with the 631G* basis set. In addition to traditional linear structure, an elongated cyclic species of C2v symmetry represents another minimum energy structure located about 9 kJ/mol above the linear one (the MP4 energy separation in the MP2 geometry is 73 kJ/mol). Calculated structure and energetics is completed with harmonic vibrational analysis including isotope effects. © 1992.
 Slanina, Z., Leś, A., & Adamowicz, L. (1992). Dimerization in the pyridone/hydroxypyridine tautomeric systems: relative stabilities of the dimers in the 2pyridone/2hydroxypyridine and in the 4pyridone/4hydroxypyridine systems. Journal of Molecular Structure: THEOCHEM, 257(34), 491498.More infoAbstract: Three dimers created from the 2pyridone/2hydroxypyridine tautomers and four dimers created by the 4pyridone/4hydroxypyridine tautomers were described using ab initio selfconsistentfield (SCF) calculation and the energies were corrected for the electron correlation contribution by using the secondorder MøllerPlesset (MP2) perturbation treatment. Three and four energy minima were revealed on the dimeric potential hypersurfaces of 2pyridone/2hydroxypyridine and 4pyridone/4hydroxypyridine, respectively. The relative stabilities of the three and fourmembered systems were evaluated in terms of partition functions constructed from the calculated dimeric parameters. The MP2 energies suggest that, under the conditions of interdimeric equilibrium, there is a coexistence of the oxooxo and hydroxyhydroxy dimers of the 2pyridone/ 2hydroxypyridine system. For the 4pyridone/4hydroxypyridine tautomeric system MP2 calculations predict a predominance of the hydroxyhydroxy dimer. © 1992.
 Adamowicz, L. (1991). Electron affinities of small linear carbon clusters. Coupled cluster calculations with firstorder correlation orbitals. The Journal of Chemical Physics, 94(2), 12411246.More infoAbstract: Electron affinities of the C3, C4, C5, and C6 linear clusters are theoretically determined using an ab initio procedure which combines the firstorder correlation orbital method and the coupled cluster method. The theoretical values 1.58, 3.41, 2.43, and 3.69 e.V. for C3, C4, C5, and C6, respectively, are systematically lower than the experimental estimates of Smalley et al. [Chem. Phys. Lett. 144, 431 (1988) ] (1.95, 3.70, 2.80, and 4.10 e.V.); however, the trends corresponding to the oddeven numbered chain alternation and to the increase of the size of the cluster are very similar. © 1991 American Institute of Physics.
 Adamowicz, L. (1991). Metastable optical excitations of linear C^{}_{5}. Chemical Physics Letters, 180(5), 466471.More infoAbstract: It is theoretically predicted that the C5 electronic vertical excitations are metastable with respect to the electron ejection. The energies of the lowest 4Σu, 4Σg, 2Σu, 2Σg, 2Δu and 2Δg states evaluated using firstorder correlation orbitals and the coupled cluster method are reported. © 1991.
 Adamowicz, L. (1991). NNOHCl complex. Ab initio calculations with the coupled cluster method and firstorder correlation orbitals. Chemical Physics Letters, 176(2), 249254.More infoAbstract: The coupled cluster method and the firstorder correlation orbital technique is applied to determine which N2OHCl configuration out of three planar equilibrium structures found in the MBPT (2) geometry optimizations is the most stable. The result agrees with the recent experimental determination of Kukolich and coworkers. © 1991.
 Adamowicz, L. (1991). Photoexcitations of the Si^{}_{4} anion. Theoretical study. Chemical Physics Letters, 185(34), 244250.More infoAbstract: Some stable and metastable (autodetaching) excited states of the Si4 anions are characterized with the use of the firstorder correlation orbital method and the coupledcluster method. There are three doublet states of the anion, X 2B2g, 2B1g and 2B1u, which lie below the electron ejection threshold. All other states have total energy higher than the energy of the neutral cluster. © 1991.
 Adamowicz, L. (1991). Stable and metastable photoexcitations of linear C^{}_{6}. Theoretical study. Chemical Physics Letters, 182(1), 4550.More infoAbstract: It is theorically predicted that there are several stable electronic excitations for the C6 cluster anion, lying below the electron ejection threshold and the photodissociation limit. The relative energies of these states are determined using the firstorder corelation orbital method and the coupledcluster method. In addition to the stable vertical excitations, we also predict some metastables states near the electron ejection level. © 1991.
 Adamowicz, L. (1991). Theoretical study of doubly charged negative ions of elemental clusters. Stability of C_{8}^{2}. Journal of Chemical Physics, 95(11), 86698670.
 Adamowicz, L. (1991). Theoretical study of optical excitations of linear C_{4}^{}. Chemical Physics, 156(3), 387394.More infoAbstract: Photoelectron vertical excitations of the C4 linear cluster are determined using the firstorder correlation orbital method and the coupled cluster method. Excitation energies for the calculated states of C4 are, respectively: 1.24 eV (2Σg+), 1.63 eV (2Σu+), 2.91 eV (2Πu), 3.45 eV (4Πu), 3.78 eV ((2) 2Πu) and 5.16 eV (2Φu). The first four excitations are below the electron ejection threshold of 3.75 eV, the fifth lies at the threshold, and the sixth is a metastable state. © 1991.
 Adamowicz, L., & Pluta, T. (1991). Metastable He2 ions formed by twoelectron attachment to the excited He2+ g+2 (1g22g1) core. Physical Review A, 44(5), 28602867.More infoAbstract: Four metastable states (1 u4, 2 u4, u4, and 4Iu), resulting from twoelectron attachments to the excited He2+ core (2g+), are characterized using the numerical HartreeFock method. It is determined that such metastable states are formed when both valence electrons are placed into equally diffused orbitals, which have bonding charter, and whose angular momentum quantum numbers do not differ by more than 1. © 1991 The American Physical Society.
 Adamowicz, L., & Pluta, T. (1991). Numerical HartreeFock study on the Rydberglike excitations of He^{}_{2}. Chemical Physics Letters, 179(56), 517523.More infoAbstract: Two new metastable states, 4Φg and 4Ig, of the He2 anion are characterized using the numerical HartreeFock method. It is determined that such metastable quartet states are formed when the He+2 core attracts two electrons, which are placed into bonding orbitals, whose angular momentum quantum number differs by one. © 1991.
 Fulara, J., Nowak, M. J., Lapinski, L., Leś, A., & Adamowicz, L. (1991). Theoretical and matrixisolation experimental study of the infrared spectra of 5azauracil and 6azauracil. Spectrochimica Acta Part A: Molecular Spectroscopy, 47(5), 595613.More infoAbstract: The infrared absorption spectra of 5azauracil and 6azauracil isolated in lowtemperature Ar and N2 matrices are reported. Within the limits of accuracy of spectroscopic measurements, both compounds appear to exist, in Ar and N2 matrices, exclusively in dioxo tautomeric forms. An assignment of the observed infrared absorption bands is proposed based on the comparison of experimental frequencies and intensities with those calculated theoretically at ab initio HartreeFock level with 631G** basis set. Infrared spectra predicted at this level of theory reproduce the experimental spectra sufficiently accurately to enable a reliable assignment. Basis set dependency (631G**, 321G) of the theoretical result was also accounted for and it turned out that for accurate prediction of frequencies and intensities the use of basis set augmented with polarization functions is of crucial importance. © 1991.
 Glass, R. S., Adamowicz, L., & Broeker, J. L. (1991). Theoretical studies on transannular S⋯S interactions in geometrically constrained 1,5dithiocane derivatives. Journal of the American Chemical Society, 113(4), X1071.More infoAbstract: This paper probes, using theoretical calculations, the nature of nonbonded interaction between sulfur functional groups in geometrically constrained 1,5dithiocane derivatives. Vibronic analysis using SCF/STO3G* ab initio calculations of the boat conformers of naphtho[1,8b,c]1,5dithiocin (1) and its 1oxide (2), 1,5dioxide (3), 1,1dioxide (4), and 1,1,5trioxide (5) were carried out. The calculated geometries are close to the experimental geometries determined by singlecrystal Xray crystallographic methods. Comparison of the frequencies of the S⋯S rocking mode for these compounds was used to test whether there is bond formation in 25. The results, using this new criterion, show that there is no significant bond formation in 2 and 3 despite the observed geometry changes ascribed to "incipient" bond formation. There is also no attractive interaction in 4 and 5. The weak attraction in 2 and 3 may be due to dipole or chargetransfer interactions.
 Kozlowski, P. M., & Adamowicz, L. (1991). An effective method for generating nonadiabatic manybody wave function using explicitly correlated Gaussiantype functions. The Journal of Chemical Physics, 95(9), 66816698.More infoAbstract: General formalism for the application of explicitly correlated Gaussiantype basis functions for nonadiabatic calculations on manybody systems is presented. In this approach the motions of all particles are correlated in the same time. The energy associated with the external degrees of freedom, i.e., the motion of the center of mass, is eliminated in an effective way from the total energy of the system. In order to achieve this, methodology for construction of the manybody nonadiabatic wave function and algorithms for evaluation of the multicenter and multiparticle integrals involving explicitly correlated Gaussian cluster functions are derived. Next the computational implementation of the method is discussed. Finally, variational calculations for a model threebody system are presented. © 1991 American Institute of Physics.
 Kurtz, J., & Adamowicz, L. (1991). Theoretical vibrations of carbon chains C_{3}, C_{4}, C_{5}, C_{6}, C_{7}, C_{8}, and C_{9}. Astrophysical Journal Letters, 370(2), 784790.More infoAbstract: The MBPT(2) procedure with the 6  31G* basis set was used to study nearly linear carbon chains. The theoretical vibrational frequencies of the molecules C3 through C9 are presented and, for C3 through C6, compared to experimental stretching frequencies and their 13C/12C isotopomers. Predictions for C7, C8, and C9 stretching frequencies are calculated by directly scaling the theoretical frequencies with factors derived from the experimentaltotheoretical ratios known for the smaller molecules.
 Leś, A., & Adamowicz, L. (1991). Firstorder correlation orbitals for the MCSCF zerothorder wave function. Chemical Physics Letters, 183(6), 483490.More infoAbstract: The originally proposed firstorder correlation orbital method (L. Adamowicz and R.J. Bartlett, J. Chem. Phys. 86 (1987) 6314) has been extended in order to include the MCSCF zerothorder wave function. The present method is based on the perturbation theory with the zerothorder Hamiltonian suggested by Andersson (J. Phys. Chem. 94 (1990) 5483). In the present work we restrict our consideration to the simplest case, i.e. the twoelectron/twoorbital MCSCF reference wave function. The procedure is tested on the model system  lithium dimer  at a variety of interatomic distances. The results show that the calculated secondorder correlation energy as well as the value of the secondorder Hylleraas functional in the reduced space of the firstorder correlation orbitals behave correctly upon dissociation of the system. © 1991.
 Leś, A., & Adamowicz, L. (1991). The correlated molecular electrostatic potential and electric field of 2 (1H)pyrimidone and 2hydroxypyrimidine. Chemical Physics, 153(3), 409414.More infoAbstract: The molecular electrostatic potential and molecular electric field have been estimated by means of the expectation values of the respective oneelectron operators. We used the molecular density matrix that includes the electron correlation effects up to the secondorder of the many body perturbation theory. The results show that around the 2(1H)pyrimidone molecule one may distinguish the electrophilic and nucleophilic regions, the latter characterized by two potential minima of 2.9 V. In the tautomeric form, 2hydroxypyrimidine, a third potential minimum of 2.1 V appears close to the N1 nitrogen atom. For both molecules strong orientational forces acting on polar solvents are predicted in the vicinity of oxygen (O7) and nitrogen (N3) atoms. The electron correlation effects do not significantly alter the SCF values of the electrostatic potential and electric field at the distances within the van der Waals envelope of the pyrimidine bases. At larger distances, however, the correlation correction is significant, particularly in the direction facing the proton transfer path. © 1991.
 MårtenssonPendrill, A., Alexander, S. A., Adamowicz, L., Oliphant, N., Olsen, J., Ster, P., Quiney, H. M., Salomonson, S., & Sundholm, D. (1991). Beryllium atom reinvestigated: A comparison between theory and experiment. Physical Review A, 43(7), 33553364.More infoAbstract: We compare the theoretical and experimental energies for the ground state of the beryllium atom and investigate possible sources for the small discrepancy of about 60 hartrees found by Bunge [Phys. Rev. A 14, 1965 (1976); 17, 486(E) (1978)]. Indications that the correlation energy may be underestimated in Bunges work have been confirmed by a recent, very extensive multiconfigurational HartreeFock (MCHF) calculation. We emphasize that the critical part of the comparison between theory and experiment is the sum of the first and second ionization energies the third and fourth are known more accurately from theory and present the theoretical results accordingly. Before a comparison with experimental results can be performed, corrections must be added to account for mass polarization, for the effect of relativity including the Breit interaction and for radiative effects. The previously unknown masspolarization contribution to the first ionization energy has recently been determined experimentally. Relativity is most important for the 1s electrons and this effect was included in Bunges work, whereas the relativistic effect on the correlation involving the 2s electrons was neglected. Here, these contributions have been calculated to leading order. A crude estimate of the contribution to the Lamb shift from the 2s electrons is also given. When the revised relativistic corrections are combined with recent results from a very extensive MCHF calculation, the discrepancy in the beryllium groundstate energy is reduced to (1050) hartrees. © 1991 The American Physical Society.
 Nowak, M. J., Lapinski, L., Fulara, J., Leś, A., & Adamowicz, L. (1991). Theoretical and infrared matrix isolation study of 4(3H)pyrimidinethione and 3(2H)pyridazinethione. Tautomerism and phototautomerism. Journal of Physical Chemistry, 95(6), 24042411.More infoAbstract: IR spectra of 4(3H)pyrimidinethione and 3(2H)pyridazinethione isolated in argon and nitrogen matrices are reported. The thiol and thione tautomeric forms of 4(3H)pyrimidinethione were found in matrices in relative concentrations ∼5:1. IR spectra of gaseous sample showed that the ratio of tautomers in matrices corresponds to the gasphase tautomeric equilibrium. Only the thione form of 3(2H)pyridazinethione was observed in matrices. UVvis irradiation of the matrices caused the conversion of the thione forms of the studied molecules into the thiol forms. The IR spectra of the thione and thiol tautomers were predicted theoretically at SCF/321G* level, and a comparison of the predicted and experimentally observed spectra enabled an assignment of most of the observed bands. © 1991 American Chemical Society.
 Oliphant, N., & Adamowicz, L. (1991). Coupledcluster method truncated at quadruples. The Journal of Chemical Physics, 95(9), 66456651.More infoAbstract: The coupledcluster (CC) equations including single, double, triple, and quadruple excitation amplitudes (CCSDTQ) are derived diagramatically, and the complete set of CCSDTQ equations are presented. These equations have been programmed and an iterative reduced linear equation method is used to solve these equations. The potential curves for the dissociation of a model system with a single bond (Li2 and LiH) is calculated using CC doubles (CCD), singles and doubles (CCSD), singles, doubles, and triples (CCSDT), and CCSDTQ. These calculations demonstrate the magnitude of the CC contributions arising from single, double, triple, and quadruple excitation amplitudes to the stretching of a chemical bond. © 1991 American Institute of Physics.
 Oliphant, N., & Adamowicz, L. (1991). Multireference coupledcluster method using a singlereference formalism. The Journal of Chemical Physics, 94(2), 12291235.More infoAbstract: A multireference coupledcluster singles and doubles method utilizing two reference determinants which differ by a two electron excitation is proposed. One of these determinants is selected as the formal reference determinant. The proposed method includes singlereference coupledcluster equations truncated after quadruples. These equations are graphically derived using Feynman diagrams. The appropriate restrictions are then placed on the triple and quadruple amplitudes to allow only those amplitudes which correspond to single and double excitations from the second reference determinant. © 1991 American Institute of Physics.
 Pauley, D. J., Roehrig, M. A., Adamowicz, L., Shea, J. C., Haubrich, S. T., & Kukolich, S. G. (1991). Microwave measurements and theoretical calculations on the structures of NNOHCl complexes. The Journal of Chemical Physics, 94(2), 899907.More infoAbstract: Pulsedbeam Fourier transform microwave spectroscopy was used to measure a and b dipole transitions for the N2OH35Cl, N 2OH37Cl, N2OD35Cl, and 15NNOH35Cl van der Waals complexes. The observed transition frequencies were fit to determine the spectroscopic constants ADK, B, C, Dj, DJK, eQqaa(Cl), and eQqbb(Cl). The structure of the complex appears to be a planar asymmetric top with a centersofmass separation Rc.m. ≈ 3.51 Å. The angle θ between Rc.m. and the HCl axis is approximately 110°. The angle φ between the N2O axis and Rc.m. is approximately 77°. The structure was fit using a weighted least squares fit to B and C isotopic rotational constants with R c.m. θ, and φ as the adjustable parameters, and this procedure yielded three local minima with standard deviations less than 5 MHz. Principal axis coordinates for the Cl, H, and terminal N atoms in the complex were determined with single isotopic Kraitchman analysis to aid in the selection of the "best" structure. In a second structural analysis R c.m. θ, and φ values were determined from the spectroscopic constants B, C, and eQqaa(Cl). The "best fit" structure parameters for N2OHCl are Rc.m. = 3.512(2) Å, θ = 110(9)°, and θ = 77(2)°. Ab initioa calculations of N2OHCl structures using GAUSSIAN86 with MP2 yielded three energetically stable equilibrium conformations. One of the bound structures is very similar to the present experimental vibrationally averaged structure. © 1991 American Institute of Physics.
 SaintMartin, H., Blake, I. O., Leś, A., & Adamowicz, L. (1991). Ab initio calculations of the pyrophosphate hydrolysis reaction. Biochimica et Biophysica Acta (BBA)/Protein Structure and Molecular, 1080(3), 205214.More infoPMID: 1659451;Abstract: Ab initio quantum mechanical calculations were used to study the hydrolysis H4P2O7 + H2O2H3PO4, as well as some molecular properties of the reactants and products. SCF calculations with several basis sets ranging from minimal to extended with polarization functions were used to look at the basis dependency of the reaction enthalpies and optimized geometries. Although the minimal basis sets yield erratic predictions of the enthalpy, when a more extended basis (321G*) was used for the geometry optimization, and the total energies of the reactants and products were computed with this and larger basis sets, we obtained more consistent predictions of the structural properties of the POP bridge and of the heat of the hydrolysis reaction (ΔE = 7.39 kcal/mol at the SCF/631G** level). A comparison is made with previous estimates performed with smaller basis sets and without taking into account the electron correlation effects, which are calculated in the present work. The inclusion of the zero point energy calculated using the harmonic approximation, and of the electronic correlation energy determined at the MBPT(2) level, raised the computed heat of the reaction to 3.83 kcal/mol, and when an estimate for the thermal energy was added, the value obtained was of 3.38 kcal/mol. In conclusion, we found that the hydrolysis of pyrophosphate should be exothermic in the gas phase. The implications of this result in relation to some recent theories about enzyme catalysis are discussed. © 1991.
 Slanina, Z., & Adamowicz, L. (1991). On the thermodynamic consequences of the threefold isomerism in the LiCN system. Thermochimica Acta, 184(1), 109116.More infoAbstract: The LiCN system which was recently characterized computationally, is considered to be an equilibrium mixture of three isomeric species (lithium isocyanide, a bridged species and lithium cyanide). While the isocyanide prevails at low temperatures, at moderate and higher temperatures the isocyanide and bridged species can exhibit comparable stability or even stability interchange. The overall standard heat capacity at constant pressure shows a clear temperature maximum; the isomeric enhancement of heat capacity can be as high as 19.5 J K1 mol1. The role of structural fluxionality is discussed briefly. © 1991.
 Slanina, Z., & Adamowicz, L. (1991). Temperature isomeric interplay in the 2pyridone2hydroxypyridine system: thermodynamically consistent treatment. Thermochimica Acta, 177(C), 101107.More infoAbstract: The tautomeric system of 2pyridone (P2) and 2hydroxypyridine (HP2) is described by partition functions supplied with molecular parameters from recent advanced computations. In contrast to other isomeric systems studied within the approach, the simple Boltzmann factors represent here a reasonable approximation to isomeric relative populations derived from thermodynamically consistent (i.e., including rotationalvibrational motions if relevant) treatment. At moderate temperatures the HP2 structure clearly prevails; however, in the high temperature limit the P2 species also becomes significant. Its presence is manifested in the isomeric enhancement of heat capacity, which at ≈ 640 K reaches its maximum value of 3.1 J K1 mol1. The relationship to interpretation or simulation of molecular spectra is discussed. © 1991.
 Adamowicz, L. (1990). Firstorder correlation orbitals for the spinunrestricted HartreeFock zeroorder wave function. Electron affinities of C_{4} clusters. The Journal of Chemical Physics, 93(9), 66856693.More infoAbstract: The secondorder Hylleraas functional and the NewtonRaphson optimization technique have been used to generate firstorder correlation orbitals (FOCOs) for the spinunrestricted HartreeFock (UHF) zeroorder wave function. The correlation orbitals are linear combinations of the UHF virtual orbitals and are different for electrons with α and β spins. We show that even the number of FOCOs is significantly reduced with respect to the number of all UHF virtual orbitals, the decrease of the secondorder correlation energy is rather small. The primary application of the FOCO UHF method is to study larger openshell molecular systems at higher correlated levels of the theory. The coupledcluster calculations with FOCOs on the electron affinities of the linear and rhombic carbon C4 clusters, presented here, illustrate the capabilities of the proposed methodology. © 1990 American Institute of Physics.
 Davidson, E. R., Feller, D., Boyle, C. M., Adamowicz, L., Clark, S. A., & Brion, C. E. (1990). HartreeFock limit and configuration interaction calculations of the ionmolecule overlap amplitude for hydrogen fluoride: Comparison with EMS experiments. Chemical Physics, 147(1), 4550.More infoAbstract: The valence orbital momentum distributions of HF have been calculated using extended Gaussian basis sets at the SCF limit and CI levels and with a numerical HartreeFock procedure. The basis sets and CI methodologies were capable of recovering approximately 85% of the estimated correlation energy. Comparison with previously published experimental results shows a disagreement with these calculations for the shape of the 1π momentum distribution, as well as a considerable difference between the measured and calculated intensity of the 2σ momentum distribution. © 1990.
 Les, A., & Adamowicz, L. (1990). Theoretical study of simple electron transfer reactions involving oxo radicals and anions. Chemical Physics Letters, 175(3), 187191.More infoAbstract: The heat of electron transfer reactions involving oxo radicals and anions is calculated using the firstorder correlation orbitals and the coupled cluster method. Despite systematic errors which remain in the theoretical electron affinities versus the experimental affinities, the agreement in the electron transfer energy is much more plausible. © 1990.
 Leś, A., & Adamowicz, L. (1990). Tautomerism of 2 and 4thiouracil. Ab initio theoretical study. Journal of the American Chemical Society, 112(4), 15041509.More infoAbstract: Ab initio quantum mechanical studies on tautomerism are extended to the 2thiouracil and 4thiouracil molecules. We considered the protomeric tautomerism in vacuum and conclude that for both species the oxothione forms should prevail in full agreement with recent experimental matrix isolation studies. Surprisingly, the next most stable structure appears to be the hydroxymercapto form. The relevance of the relative stability order of 2thiouracil and 4thiouracil to the tautomerism of various derivatives is discussed. Major features of the vibrational spectra of various tautomeric forms are presented and compared with the experiment.
 Leś, A., & Adamowicz, L. (1990). Theoretical ab initio study of the protomeric tautomerism of 2hydroxypyrimidine, 4hydroxypyrimidine, and their derivatives. Journal of Physical Chemistry, 94(18), 70217032.More infoAbstract: We investigated the gasphase protomeric tautomerism of a series of monosubstituted derivatives of two parent compounds, 2hydroxypyrimidine and 4hydroxypyrimidine (substituted at the C(4) and C(2) positions, respectively, by H, NH2, OH, SH, and SCH3). We used the ab initio HartreeFock method, the manybody perturbation theory (MBPT), and the coupled cluster (CC) method with the doubleζ Gaussian basis augmented with polarization functions to calculate the electronic contribution to the molecular total energy. The energy of the zeropoint nuclear vibrations was obtained within the harmonic approximation by means of the SCF/321G* analytical energy derivatives. For each molecule we considered several tautomeric forms corresponding to different protonation sites: at the ring nitrogen atoms and the exocyclic oxygen atom. We estimated the relative temperaturedependent distribution of various tautomeric forms in the gas phase and predicted the environmental influence on the tautomeric equilibrium. We show that the commonly applied MBPT(2) theory for the evaluation of the electron correlation effects should be supplemented by an estimation of the higher order electron correlation effects. For all investigated systems, they appear to favor the oxo tautomeric forms by 14 kJ mol1. In all cases studied in the present paper we achieved a qualitative agreement with the conclusions based on the recent lowtemperature matrixisolation IR spectra. We lend support to the experimentally derived conclusion that 2hydroxypyrimidine should exist in the gas phase (or in the weakly polar environment) almost exclusively in the hydroxy tautomeric form. The theoretically derived equilibrium constant is equal to K(oxo/hydroxy) = 0.01 at T = 500 K. A coexistence of the hydroxy and oxo forms, with a clear predominance of the hydroxy form, should characterize the cytosine (K = 0.37), isocytosine (K = 0.15), and Smethylated 4thiouracil (K = 0.17) vapors. A nearly equimolar hydroxy:oxo mixture should correspond to the gasphase 4hydroxypynmidme (K = 1.04) and Smethylated 2thiouracil (K = 1.08). According to our estimation of the environment effects, the hydroxyoxo tautomeric equilibrium should be strongly shifted toward the oxo form in polar solvents. The tautomeric rearrangement corresponding to proton transfer between the ring nitrogen atoms (N(1)H to N(3)H and vice versa) appears to be highly unfavorable in the gas phase. On the other hand, such a process can easily occur in polar solvents due to an additional strong (mostly electrostatic) solvent stabilization of the energetically unfavored tautomeric form. © 1990 American Chemical Society.
 Leś, A., & Adamowicz, L. (1990). Which tautomeric form of 5methylcytosine predominates in vacuum?. Journal of Molecular Structure, 221(C), 209218.More infoAbstract: An application of highlevel ab initio quantum chemistry methods to study the tautomeric equilibria of 5methylcytosine in the gasphase is described. Calculations for the hydroxyamino, oxoamino and oxoimino forms have been performed at the secondorder level of the many body perturbation theory (MBPT(2)) with a doublezeta basis set augmented with polarization functions. The optimized geometry and vibrational modes were obtained with the SCF/321G method. The calculations clearly show that the most stable tautomeric form in vacuum is hydroxyamino. The remaining two forms, i.e. oxoamino and oxoimino, are each estimated to be less stable than the main form by about 15 kJ mol1. © 1990.
 Nowak, M. J., Lapinski, L., Rostkowska, H., Leś, A., & Adamowicz, L. (1990). Theoretical and matrixisolation experimental study on 2(1H)pyridinethione/2pyridinethiol. Journal of Physical Chemistry, 94(19), 74067414.More infoAbstract: Tautomerism and infrared spectra of 2(1H)pyridinethione were studied with theoretical and experimental methods. The relative stabilization energy of thiol and thione tautomers was calculated by using the SCF + MBPT(2) method of quantum chemistry with DZP basis set. The thiol form was predicted to be predominant in the gas phase. Vibrational frequencies and intensities of the IR bands were calculated at the SCF 321G* level. In reported experimental IR spectroscopic studies of Ar and N2 matrixisolated compounds, the thiol and thione tautomers were found in relative concentrations of about 30:1. The phototautomeric reaction  the conversion of the thione form into the thiol form after UVvis irradiation of the matrix  was observed and used to separate the spectra of the two forms. Comparison of the experimental IR spectra of matrixisolated and crystalline compounds with the spectra predicted theoretically gave an assignment for the majority of the absorption bands in the spectra of both tautomers. © 1990 American Chemical Society.
 Oliphant, N., & Adamowicz, L. (1990). Coupled cluster calculations for the BC molecule using numerical correlation orbitals. Chemical Physics Letters, 168(2), 126130.More infoAbstract: Calculations were performed on the BC molecule using a method which combines the numerical MCSCF technique and the coupled cluster method involving single, double and triple excitations (CCSD+T(CCSD) level) in an attempt to determine, very accurately, re in the ground electronic state. The result of 2.817 au agrees quite well with one of the experimentally suggested values which is 2.820 au. © 1990.
 Adamowicz, L. (1989). Comment on: "Determination of the relative stability of fluorocytosine tautomers with a single and double excitation coupledcluster (CCSD) method". Chemical Physics Letters, 156(6), 628.More infoAbstract: The title manuscript omitted a number of important acknowledgements. These are now presented, in order to give proper credit to the contributions of W.B. Person, A. Jaworski and coworkers. © 1989.
 Adamowicz, L. (1989). Dipolebound anionic state of nitromethane. Ab initio coupled cluster study with firstorder correlation orbitals. The Journal of Chemical Physics, 91(12), 77877790.More infoAbstract: The theoretical procedure based on the coupled cluster method and the firstorder correlation orbital for studying dipolebound states of polyatomic molecules is presented. The calculation of the lowest dipolebound state for the nitromethane molecule indicates that the molecular dipole of this molecule, which is experimentally known to be 3.46 D, can attract an extra electron with the energy of 0.000 127 hartree and form a bound state. This state should be considered as a shortliving intermediate leading to the formation of a stable valencebound anion. © 1989 American Institute of Physics.
 Adamowicz, L. (1989). Electronic structure of polyatomic systems determined with firstorder correlation orbitals. Very accurate calculations on 10electron molecules. Journal of Physical Chemistry, 93(5), 17801784.More infoAbstract: A recently proposed method for generating the firstorder correlation orbital space1 is employed with large Gaussian basis sets to calculate the electronic correlation energy for small polyatomic molecules with manybody perturbation theory and the coupled cluster method. Different sizes of the correlation orbital spaces are considered, and comparison is made with approximate natural orbitals generated by diagonalization of the virtualvirtual part of the secondorder density matrix. © 1989 American Chemical Society.
 Adamowicz, L. (1989). Tautomeric equilibria of 2pyridone/2hydroxypyridine. Ab initio coupled cluster calculations with firstorder correlation orbitals. Chemical Physics Letters, 161(1), 7378.More infoAbstract: Tautomeric equilibria of the 2pyridone/2hydroxypyridine are studied using the coupledcluster method with single, double, and triple excitations. The 0 K total energy difference is estimated to be 3.5 kcal/mol in favor of the hydroxy form. © 1989.
 Adamowicz, L., & Frum, C. I. (1989). Electronic structure of polyatomic systems determined with firstorder correlation orbitals. Coupled cluster calculations on lithium cyanide. Chemical Physics Letters, 157(6), 496500.More infoAbstract: Firstorder correlation orbitals and the coupled cluster method were employed together with an extended Gaussian basis set to evaluate the relative energies of three isomers of lithium cyanide. The most stable linear LiNC structure has an energy 0.75 kcal/mol lower than the Tshape structure and 2.34 kcal/mol lower than the linear LiCN structure. The first result remains in accord with the recently established lower limit of 0.34 kcal/mol. © 1989.
 Adamowicz, L., & Kurtz, J. (1989). Theoretical studies of C_{5} with firstorder correlation orbitals and the coupled cluster method. Chemical Physics Letters, 162(45), 342348.More infoAbstract: The manybody perturbation theory and the coupled cluster method with firstorder correlation orbitals are used to study the structure of the linear C5 molecule. Calculations produce the equilibrium geometry with the outer and inner bond lengths equal to 1.294 and 1.287 Å, respectively. This corresponds to the equilibrium rotation constant, Be, of 0.08442 cm1. © 1989.
 Glass, R. S., Adamowicz, L., & Broeker, J. L. (1989). Theoretical studies on transannular SS interactions in geometrically constrained 1,5dithiocane derivatives. Journal of Molecular Structure: THEOCHEM, 186(C), 273291.More infoAbstract: Vibronic analysis on naphtho[1,8b,c]1,5dithiocin (10) and naphtho[1,8b,c]1,5dithiocin1oxide (11) using the HartreeFock method with the STO3G basis set shows no evidence for covalentbond formation in 11 compared with 10. Thus the transannular interaction in 11 is due to electrostatic interaction and not incipient sulfurane formation. Examination of proposed intermediates in the reaction of 11 to form dication 12 using models and ab initio HartreeFock calculations using a 631G* basis set provides insight into this reaction. The calculations gave no indication of sulfurane formation. © 1989.
 Leś, A., & Adamowicz, L. (1989). Oxohydroxy tautomerism of uracil and 5fluorouracil. Journal of Physical Chemistry, 93(20), 70787081.More infoAbstract: The three lowest energy tautomeric forms of uracil and 5fluorouracil have been studied by using the secondorder manybody perturbation theory (MBPT(2)) with Gaussian DZP basis sets. The zeropoint nuclear energy has been estimated by means of the harmonic approximation using analytical derivatives of the SCF 321G energy. The uracil and 5fluorouracil molecules have been predicted to exist in the gas phase in the dioxo forms in agreement with the majority of experimental data. The 2hydroxy4oxo and 2oxo4hydroxy tautomeric forms of uracil have been found to be less stable than the main dioxo form by 44 and 50 kJ mol1, respectively. The corresponding values for 5fluorouracil are 29 and 52 kJ mol1. The relevance of the present results to recent IR and fluorescence spectroscopical experimental works is discussed. Some aspects of the point mutation theory in view of the present work are also considered. © 1989 American Chemical Society.
 Leś, A., Adamowicz, L., & Bartlett, R. J. (1989). Relative stability of cytosine tautomers with the coupled cluster method and firstorder correlation orbitals. Journal of Physical Chemistry®, 93(10), 40014005.More infoAbstract: Three tautomeric forms of cytosine molecule have been studied by the coupled cluster method with single, double, and triple excitations using firstorder correlation Orbitals and Gaussian DZP basic sets. The zeropoint nuclear energy has been estimated by the SCF 321G method with analytical first and second derivatives of the total energy. The hydroxyamino tautomer has been predicted to predominate in the gas phase in agreement with recent experimental data. The oxoamino and oxoimino forms are less stable than the hydroxyamino tautomer by 4.2 and 6.4 kJ mol1, respectively. The pertinence of the present results to the interpretation of the experimental IR spectra is discussed. © 1989 American Chemical Society.
 Nan, Y. u., & Adamowicz, L. (1989). Numerical multiconfiguration selfconsistentfield study of the hyperfine structure in the infrared spectrum of ^{3}He^{4}He^{+}. The Journal of Chemical Physics, 90(8), 43924400.More infoAbstract: Numerical multiconfiguration selfconsistentfield (MCSCF) procedure is employed to calculate the hyperfine interaction energy for the 3He4He+ cation at different internuclear separations. A conventional vibrational averaging of the energy allows the calculation of hyperfine splitting in the IR spectrum. This is done for several of the lowest vibrational states. We predict that the hyperfine splitting will get larger with the increasing vibrational excitation. Various different MCSCF wave functions are used in the study to verify the convergence of the hyperfine parametes and to determine the importance of the electronic correlation. © 1989 American Institute of Physics.
 Nan, Y. u., Wing, W. H., & Adamowicz, L. (1989). Hyperfine structure in the infrared spectrum of He3 He+4. Physical Review Letters, 62(3), 253256.More infoAbstract: In our recent experiment, the hyperfine splittings of He3 He+4 were observed in the infrared spectrum. Now we present an ab initio quantummechanical calculation of hyperfine structure in the (10) vibrational band of the He3 He+4 ground electronic state. The hyperfine interaction coefficients b, c, and γ are calculated by using the numerical multiconfiguration selfconsistentfield method at a set of closely spaced internuclear distances and then averaged over nuclear coordinates for individual vibrationrotation states. The theoretical spectra agree well with the experimental observations. © 1989 The American Physical Society.
 Pluta, T., Bartlett, R. J., & Adamowicz, L. (1989). Metastable He2 and its autodetachment spectra: An accurate coupledcluster study. Physical Review A, 40(5), 22532259.More infoAbstract: The unusual autodetachment spectrum of metastable He2 is studied theoretically, using highlevel coupledcluster methods and hybrid numerical and Slater orbital basis sets. By obtaining accurate curves for the repulsive wall of the ground He2(X g+1) state, the excited He2(a u+3) state, and the metastable He2(4 g) state, we are able to provide an alternative explanation for the experimental observations, which had cast doubt on the veracity of the accepted curve for the repulsive part of the He2(X1g+) potential. We attribute the experimental peak at 15.78±0.13 eV to transitions of vibrationally excited states (=2 and higher) of He2 to the He2(X g+1) continuum, since the v=0 transition would have a value 1 eV less. An error of this size is considered to be far outside the error bars for highly accurate correlated ab initio calculations. The electron affinity of the g4 state of the anion (measured relative to a u+3) is computed to be 0.201 and 0.212 eV for different basis sets, compared to an experimental value of 0.175±0.032 eV. © 1989 The American Physical Society.
 Adamowicz, L. (1988). Determination of the relative stability of fluorocytosine tautomers with single and double excitation coupledcluster (CCSD) method. Chemical Physics Letters, 153(23), 147152.More infoAbstract: Firstorder correlation orbitals are generated for different tautomeric forms of the 5fluorocytosine molecule and used to perform coupledcluster calculations. It is determined that the population of the most stable hydroxyamino tautomer should outnumber that of the next most stable ketoamino form by a factor of 2500 in the gas phase at room temperature. © 1988.
 Adamowicz, L. (1988). Numerical multiconfiguration selfconsistent field study of the total (electronic and nuclear) parallel polarizability and hyperpolarizability for the OH, OH^{+}, OH^{}. The Journal of Chemical Physics, 89(10), 63056309.More infoAbstract: Vibrational contributions to the molecular polarizability and hyperpolarizabilities are calculated for OH, OH+, and OH for an electric field directed along the bond axis. It was found that vibrational averaging of the polarizability works reasonably well for OH and OH, but fails for OH+, giving a result 16% to low. For the hyperpolarizabilities, the vibrational averaging strongly underestimates the vibronic contribution, whose size can exceed the electronic contribution, and therefore disqualifies this procedure for evaluation of these types of higherorder molecular properties. © 1988 American Institute of Physics.
 Adamowicz, L., & Bartlett, R. J. (1988). Excited state electron affinities of NaF, LiCl, and NaCl. The Journal of Chemical Physics, 88(1), 313316.More infoAbstract: Numerical HartreeFock and coupledcluster (CCSD) predictions of the existence of excited states of dipole bound anions are reported for NaF, LiCl, and NaCl. The calculations use a composite numerical and Slater orbital basis set. The second dipole states are found to be bound by 0.012, 0.009, and 0.021 eV for NaF, LiCl, and NaCl, respectively, and should be amenable to experimental observation. © 1987 American Institute of Physics.
 Adamowicz, L., & Bartlett, R. J. (1988). Very accurate correlated calculations on diatomic molecules with numerical orbitals: The hydrogen fluoride molecule. Physical Review A, 37(1), 15.More infoAbstract: Numerical orbital coupledcluster calculations on the FH molecule are reported, which obtain 99% of the possible correlation energy. Instead of requiring explicit r12 functions, it is demonstrated that a oneparticle but very wellchosen numerical orbital basis set is adequate to recover essentially all effects of electron correlation. Further reduction of the dimension of the numerical orbital basis set is accomplished via our recently proposed optimized virtualorbitalspace technique. Augmented by the exact secondorder energy, the latter allows a reduction to as small as 25% of the original space with no appreciable errors in the correlation energy. © 1988 The American Physical Society.
 Adamowicz, L., Bartlett, R. J., & Sadlej, A. J. (1988). Optimized virtual orbital space for highlevel correlated calculations. II. Electric properties. The Journal of Chemical Physics, 88(9), 57495758.More infoAbstract: The optimized virtual orbital space (OVOS) technique recently proposed for highlevel correlated calculations of energy surfaces, is shown to be nearly as efficient for electric field properties. In particular, the polarizability of F, and the dipole moment and polarizability of FH as a function of internuclear separation are studied. A reduction of the virtual space to about onehalf has a negligible effect on the dipole moment and polarizabilities for FH and F examples. A further reduction to onequarter is reliable when augmented with the exact secondorder result, obtained as a byproduct of the OVOS generation. This enables the extension of highlevel correlated methods to systems at least 24 times larger than those that could be accurately studied using the full space of virtual orbitals. © 1988 American Institute of Physics.
 Adamowicz, L., Bartlett, R. J., Kwiatkowski, J. S., & Person, W. B. (1988). Theoretical study of PO and PO^{}. Theoretica Chimica Acta, 73(23), 135145.More infoAbstract: The diatomic systems, PO and PO are studied, using numerical HartreeFock (NHF) and coupledcluster calculations. The latter employs a hybrid NHF and Slater orbital basis set. Highly accurate CCSD methods predict bond lengths accurate to
 Adamowicz, L., & Bartlett, R. J. (1987). Optimized virtual ortibal space for highlevel correlated calculations. The Journal of Chemical Physics, 86(11), 63146324.More infoAbstract: The second order Hylleraas functional and a NewtonRaphson orbital optimization techniquehave been used to generate an active, optimized virtual orbital space (OVOS) of substantially reduced dimension for correlated calculations. Numerical examples for CH2(1A1), C6H6, and potential curves for B2H6 and H2O2 using MBPT and coupledcluster theory demonstrate that most of the correlation energy can be obtained with a much smaller number of optimized virtual orbitals, and effectively ∼100% of the correlation energy if the OVOS result is combined with the exact secondorder energy that is evaluated as a byproduct of the OVOS generation. This suggests a potentially wide applicability of the OVOS method in high accuracy quantum mechanical calculations. © 1987 American Institute of Physics.
 Adamowicz, L., & Bartlett, R. J. (1986). Accurate numerical orbital MBPT/CC study of the electron affinity of fluorine and the dissociation energy of hydrogen fluoride. The Journal of Chemical Physics, 84(12), 68376839.More infoAbstract: The classic problem of the dissociation energy of FH and the electron affinity of F have been studied using our recent proposed numerical orbital coupledcluster (CC) method. With 105 orbitals, 96.6% of the correlation energy of FH is obtained, and exact agreement with the experimental dissociation energy. Similarly, the electron affinity of F is found to be 3.37 eV compared to an experimental value of 3.399 eV. © 1986 American Institute of Physics.
 Adamowicz, L., & Bartlett, R. J. (1986). Coupled cluster calculation of electron affinities of LiF. Chemical Physics Letters, 129(2), 159164.More infoAbstract: The three lowest states of LiF have been studied using a combination of numerical HartreeFock orbitals and Slatertype orbitals as a basis set for coupled cluster calculations. The ground state electron affinity is 0.31 eV, while in the first excited state of the ion the extra electron is bound by 0.38 × 102 eV and in the next state by 0.43 × 104 eV. The last value differs from Garrett's nonadiabatic estimate of 0.12 × 104 eV, suggesting a breakdown of the BornOppenheimer approximation. © 1986.
 Adamowicz, L., & Bartlett, R. J. (1986). Erratum: Numerical coupled HartreeFock study of the total (electronic and nuclear) parallel polarizability and hyperpolarizability for the FH, H _{2}^{+}, HD^{+}, and D_{2} molecules (The Journal of Chemical Physics (1986) 84 (4988)). The Journal of Chemical Physics, 86(12), 7250.
 Adamowicz, L., & Bartlett, R. J. (1986). Numerical coupled HartreeFock study of the total (electronic and nuclear) parallel polarizability and hyperpolarizability for the FH, H_{2}^{+}, HD^{+}, and D_{2}^{+} molecules. The Journal of Chemical Physics, 84(9), 49884991.More infoAbstract: Polarizabilities and hyperpolarizabilities of molecules have both electronic and vibrational contributions. The latter can be important in experimental determinations involving static electric fields, as in the Kerr effect. This paper reports purely numerical coupled HartreeFock calculations for the electronic states of FH. Electronic energy surfaces obtained for different electric field strengths are then used to solve, numerically, the nuclear equation for several vibrational states. The vibrational contributions to βzzz exceed the electronic component and change the sign of the total hyperpolarizability. γzzzz is similarly increased by an order of magnitude. It is also shown that a BornOppenheimer treatment of H2+ for Yzzzz is in perfect agreement with a recent fully nonadiabatic three particle solution. © 1986 American Institute of Physics.
 Salter, E. A., Adamowicz, L., & Bartlett, R. J. (1986). Comment on MBPT/CC nickel calculations. Chemical Physics Letters, 130(12), 152153.
 Adamowicz, L., & Bartlett, R. J. (1985). Coupled cluster calculations with numerical orbitals for excited states of polar anions. The Journal of Chemical Physics, 83(12), 62686274.More infoAbstract: A recently proposed technique, which combines numerical orbitals for diatomic systems with coupled cluster methods, has been applied to study excited states of negative ions of polar molecules. Computational strategy is presented and calculations of ground and excited state electron affinities are reported for the LiH and BeO molecules. LiH and BeO are found to have excited state electron affinities of 0.0028 and 0.0234 eV. Both excited state affinities should be amendable to experimental determination. © 1985 American Institute of Physics.
 Adamowicz, L., Bartlett, R. J., & McCullough Jr., E. A. (1985). Towards Numerical Solutions of the Schrödinger Equation for Diatomic Molecules. Physical Review Letters, 54(5), 426429.More infoAbstract: Manybody perturbation and coupledcluster methods using numerical HartreeFock and correlating orbitals are applied to diatomic molecules for the first time. For LiH, the correlation energy is within 0.001 a.u. of the nonrelativistic correlation energy limit, which provides an energy of 8.069 a.u. compared to the exact result of 8.070. These results suggest that highly accurate results for diatomic molecules may be obtained by the proposed approach. © 1985 The American Physical Society.
 Salter, E. A., Adamowicz, L., & Bartlett, R. J. (1985). Coupled cluster and MBPT study of nickel states. Chemical Physics Letters, 122(12), 2328.More infoAbstract: Full fourthorder MBPT calculations on the 3F, 3D and 1S states of the nickel atom with a Gaussian basis do not yield the correct nonrelativistic energy ordering. Coupledcluster single and double excitation calculations reveal a large discrepancy between fourthorder and infiniteorder results. We achieve an improved separation of the states by employing a Slater basis and introducing infiniteorder effects of triple excitations with our CCSD + T(CCSD) model. © 1985.
 Adamowicz, L., & Bartlett, R. J. (1984). Extended floating spherical Gaussian basis sets for Molecules. FSGO basis for use in advanced correlated calculations of electronic structures. Chemical Physics Letters, 110(4), 365368.More infoAbstract: The transformation of occupied and excited SCF orbitals expressed in Cartesian Gaussian form to a smaller, simpler set of floating spherical Gaussians is described. Illustrative applications at the correlated coupledcluster level are presented for Lill and H2O. © 1984.
 Adamowicz, L., & Bartlett, R. J. (1984). Extended floating spherical Gaussian basis sets for molecules. Alternative correlating orbitals for molecular energy calculations. Chemical Physics Letters, 110(4), 361364.More infoAbstract: A procedure previously described for representing large basis SCF results in terms of a smaller floating spherical Gaussian orbital (FSGO) basis set is generalized to apply to the virtual orbitals from the SCF calculation. This provides a method for systematically reducing the dimensions of the virtual space or replacing the virtual orbitals with a simpler, compact basis set. The method is illustrated by application to Lill. © 1984.
 Adamowicz, L., & Bartlett, R. J. (1984). Extended floating spherical gaussian basis sets for molecules. Generation procedure and result for H_{2}O. Chemical Physics Letters, 105(2), 167170.More infoAbstract: A procedure is proposed to generate extended floating spherical gaussian orbital (FSGO) basis sets for molecular SCF calculations by projecting large basis set SCF results onto FSGOs. This replaces the need for repeated evaluation of energy integrals and SCF iterations for extensive nonlinear optimizations of FSGOs. © 1984.
 Adamowicz, L., & McCullough Jr., E. A. (1984). High excited states of dipolebound anions of polar diatomic molecules. Chemical Physics Letters, 107(1), 7276.More infoAbstract: Stationary nucleus. Koopmans' theorem level calculations of the electron binding energies in excited state anions of several polar diatomic molecules have been performed. Binding energies as small as 5 × 108 eV (for the fifth) excited state of MgO) were obtained. The results indicate that catastrophic breakdown of the BornOppenheimer approximation should be observable in the second excited state of LiF. © 1984.
 Adamowicz, L., & McCullough Jr., E. A. (1984). Numerical multiconfiguration selfconsistentfield calculations on the first excited state of LiH^{}. Journal of Physical Chemistry, 88(10), 20452048.More infoAbstract: We have carried out numerical multiconfiguration selfconsistentfield (MCSCF) calculations on the first excited state of LiH in order to investigate the importance of electron correlation in excited states of anions of highly polar molecules. A discussion of the configuration selection and orbital optimization procedures is given. Comparison calculations were carried out on LiH so that an estimate of the electron binding energy could be made. Our best estimate, 95 μhartrees, is compared with the pseudopotential value obtained by Garrett. Some possible general conclusions regarding electron correlation in excitedstate anions of this type are presented. © 1984 American Chemical Society.
 Adamowicz, L., & Zielenkiewicz, W. (1983). Thermal curve interpretation by spectral resolution into a basic set of rectangular pulse curves  II. Modification of the algorithm and analysis of the efficiency of the method. Journal of Thermal Analysis, 26(2), 217223.More infoAbstract: An improvement of the algorithm of the spectral resolution method for the determination of thermokinetics is presented. With the aid of the modified computer program and an experimental thermal curve, an analysis of the method has been performed. © 1983 Wiley Heyden Ltd., Chichester and Akadémiai Kiadó, Budapest.
 Adamowicz, L., & Zielenkiewicz, W. (1983). Thermal curve interpretation by spectral resolution into a basic set of rectangular pulse curves  III. Resolution into other pulse curves. Journal of Thermal Analysis, 26(1), 3947.More infoAbstract: A generalized spectral resolution method for thermokinetic determination is presented and tested. Any kind and number of identification thermal curves can be used in a single calculation. © 1983 Wiley Heyden Ltd., Chichester and Akadémiai Kiadó, Budapest.
 Adamowicz, L. (1981). Thermal curve interpretation by spectral resolution into the basic set of rectangular pulse curves. Journal of Thermal Analysis, 22(2), 199204.More infoAbstract: A new numerical method for the determination of thermokinetics is proposed. This is based on the spectral resolution of a thermal curve into the basic set of unit rectangular pulse curves orthogonalized by Löwdin's transformation. A numerical example shows that the present method can be successfully applied to experimental curves. © 1981 Wiley Heyden Ltd., Chichester and Akadémiai Kiadó, Budapest.
 Adamowicz, L., & McCullough Jr., E. A. (1981). A numerical multiconfiguration selfconsistentfield method for diatomic molecules. The Journal of Chemical Physics, 75(5), 24752476.
 Jaszuński, M., & Adamowicz, L. (1981). Generation of a basis set for the calculation of molecular magnetic properties. Chemical Physics Letters, 79(1), 133136.More infoAbstract: Completeness conditions have been applied in the theory of molecular magnetic properties to determine the best gauge for a given basis set. They can similarly be used to optimise the basis for a fixed gauge origin. Test results indicate that the gauge dependence of the calculated properties can be significantly reduced in this way. © 1981.
 Szalewicz, K., Adamowicz, L., & Sadlej, A. J. (1979). Molecular electric polarizabilities. CI and explicitly correlated electricfieldvariant functions. Calculation of the polarizability of H_{2}. Chemical Physics Letters, 61(3), 548552.More infoAbstract: The concept of the socalled electricfieldvariant (EFV) functions (functions with explicit dependence on the external electric field strength) is applied within the CI method. A similar treatment of the electricfield perturbation is also proposed for gaussian geminals. Both methods are illustrated by calculation of the static electric dipole polarizability of H2. With variation of a single parameter, which enters both the perturbed function and the secondorder perturbed energy, accurate polarizability values can be obtained without any explicit extension of the set of functions employed for the unperturbed system. © 1979.
 Adamowicz, L., & Sadlej, A. J. (1978). Perturbation calculation of molecular correlation energy using Gaussiantype geminals. Secondorder pair energies of LiH and BH. The Journal of Chemical Physics, 69(9), 39924000.More infoAbstract: The secondorder pair energies of LiH and BH are calculated using RayleighSchroedinger perturbation theory. The firstorder perturbed pair functions are expanded in terms of explicitly correlated Gaussian functions. The nonlinear parameters entering the Gaussian geminals are optimized with reference to crude SCF functions according to the method proposed previously. The final values of the secondorder pair energies are then calculated using accurate SCF orbitals. At this final stage only the linear parameters are reoptimized. The calculated secondorder correlation energies of LiH and BH are compared with recent diagrammatic manybody perturbation theory results. The basis sets composed of two optimized Gaussian geminals for each spinadapted pair function are shown to give quite reliable secondorder correlation energies. The results obtained with four geminals for each pair function are superior to the most accurate manybody perturbation theory data. It is stressed that the nonlinear parameters of Gaussian geminals can be given a simple physical interpretation which facilitates their reasonable guess. The firstorder pair functions represented in terms of Gaussian geminals have a very attractive compact and simple form. If properly optimized they can also provide highly accurate values of the secondorder molecular correlation energies. © 1978 American Institute of Physics.
 Adamowicz, L., & Sadlej, A. J. (1977). Gaussian geminal basis set optimization with crude SCF reference state. Chemical Physics Letters, 48(2), 305310.More infoAbstract: Gaussian geminal basis functions for secondorder correlation energy calculations according to the Sinanoǧlu method are optimized with reference to rather crude SCF functions. The optimized geminal basis set is then used in a onestep calculation of the correlation energy with respect to the nearHartreeFock reference State. The numerical results for the beryllium atom indicate the usefulness of the proposed technique. © 1997.
 Adamowicz, L., & Sadlej, A. J. (1977). Perturbation calculation of molecular correlation energy using Gaussiantype geminals. Second And thirdorder pair energy of H_{2}. The Journal of Chemical Physics, 67(9), 42984300.