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  • 1
    Electronic Resource
    Electronic Resource
    The relativistic Dirac–Coulomb–Fock effect on atomization energies (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 7123-7126 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have used a second-order perturbation treatment of the Dirac–Coulomb–Hartree–Fock method to estimate relativistic contributions to energies in the G2/97 test set. The one-electron relativistic effect on atomization energies of molecules containing first- and second-row atoms nearly always reduces binding. When the relativistic corrections are included in G3 theory and assessed on the G2/97 test set, there is little change in overall performance. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478615
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  • 2
    Electronic Resource
    Electronic Resource
    Electron correlation in chemical bonds (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 4014-4019 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We study the correlation energy in the H2 molecule as a function of bond distance and the electron coupling parameter that modifies the strength of electron–electron repulsion. For the unscaled electron repulsion we find an unusual complexity of the correlation energy dependence on the bond distance. This complexity remains if the electron repulsion is scaled down. We pay particular attention to the range of bond distances near the H2 equilibrium geometry. We confirm that good performance of the modern exchange–correlation functionals in molecular geometry optimization is due to the cancellation of errors between the exchange and correlation components. These components have different functional dependence on the electron coupling parameter, so the cancellation of errors no longer occurs when the electron repulsion is scaled down. This observation is important if the adiabatic connection theorem is used to construct the exchange–correlation functionals. It is also relevant for the performance of exchange–correlation functionals to study ionic interactions. The performance of the correlation operator in the molecular environment is also investigated and discussed. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480950
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  • 3
    Electronic Resource
    Electronic Resource
    6-31G* basis set for atoms K through Zn (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 1223-1229 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Medium basis sets based upon contractions of Gaussian primitives are developed for the third-row elements K through Zn. The basis functions generalize the 6-31G and 6-31G* sets commonly used for atoms up to Ar. They use six primitive Gaussians for 1s, 2s, 2p, 3s, and 3p orbitals, and a split-valence pair of three and one primitives for valence orbitals, which are 4s and 5p for atoms K and Ca, and 4s, 4p, and 3d for atoms Sc through Zn. A 6-31G* set is formed by adding a single set of Gaussian polarization functions to the 6-31G set. They are Cartesian d-functions for atoms K and Ca, and Cartesian f-functions for atoms Sc through Zn. Comparison with experimental data shows relatively good agreement with bond lengths and angles for representative vapor-phase metal complexes. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476673
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  • 4
    Electronic Resource
    Electronic Resource
    Energies of C2H5O and C2H5O+ isomers (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 3292-3300 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Gaussian-2 (G2) theoretical procedure, based on ab initio molecular orbital theory, is used to calculate the energies of C2H5O and C2H5O+ isomers. The ethoxy radical cation (CH3CH2O+) is found to have a 3A‘ ground state while the singlet state is predicted to be unstable to rearrangement. The G2 adiabatic ionization potential of ethoxy radical is 10.32 eV in good agreement with a new ionization potential reported by Ruscic and Berkowitz of 10.29±0.08 eV from photoionization studies. The 2-hydroxyethyl radical (CH2CH2OH) has three isomers of nearly equal energy (within 1.6 kcal/mol). No stable 2-hydroxyethyl cation was located as it collapses to either O-protonated oxirane or 1-hydroxyethyl cation. The G2 appearance potential of CH3CHOH+ from ethanol of 10.79 eV is in excellent agreement with the recent photoionization value of 10.801±0.005 eV. The α(C–H), β(C–H), and O–H bond dissociation energies of ethanol are 94.9, 101.9, and 104.6 kcal/mol, respectively. The G2 result for the O–H bond energy is in good agreement with experiment while the results for the other two bond energies suggest that the experimental values for these quantities may be low. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468640
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  • 5
    Electronic Resource
    Electronic Resource
    Molecular orbital methods in organic chemistry (1970)
    s.l. : American Chemical Society
    Accounts of chemical research 3 (1970), S. 217-223 
    Details
    ISSN: 1520-4898
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ar50031a002
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  • 6
    Electronic Resource
    Electronic Resource
    Theoretical study of structures and energies of acetylene, ethylene, and vinyl radical and cation (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 7405-7409 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A systematic study of the structures and energies of acetylene, ethylene, and vinyl radical and cation using high levels of ab initio molecular orbital theory, including correlation effects beyond fourth order perturbation theory, is presented. In agreement with previous theoretical studies, the present theory predicts that the bridged form of C2H+3 is more stable than the classical form with an energy difference of 3.1 kcal/mol (including zero point energies). The bond dissociation energy of C2H4,D0(C–H), is calculated to be 110.2 kcal/mol in agreement with the recent photoionization study of Berkowitz et al. and in significant disagreement with several previous experimental studies. Finally, triplet vinyl cation is studied for the first time and is calculated to be 54.5 kcal/mol higher in energy than the singlet ground state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454303
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  • 7
    Electronic Resource
    Electronic Resource
    Quadratic configuration interaction. A general technique for determining electron correlation energies (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 5968-5975 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A general procedure is introduced for calculation of the electron correlation energy, starting from a single Hartree–Fock determinant. The normal equations of (linear) configuration interaction theory are modified by introducing new terms which are quadratic in the configuration coefficients and which ensure size consistency in the resulting total energy. When used in the truncated configuration space of single and double substitutions, the method, termed QCISD, leads to a tractable set of quadratic equations. The relation of this method to coupled-cluster (CCSD) theory is discussed. A simplified method of adding corrections for triple substitutions is outlined, leading to a method termed QCISD(T). Both of these new procedures are tested (and compared with other procedures) by application to some small systems for which full configuration interaction results are available.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453520
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  • 8
    Electronic Resource
    Electronic Resource
    Scalar relativistic effects on energies of molecules containing atoms from hydrogen through argon (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 718-725 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Stationary direct perturbation theory is used to calculate a scalar relativistic correction to the species in the G3/99 test set. We observe that the relativistic energy almost always reduces atomization energies, electron affinities, and ionization potentials. Exceptions occur when s orbitals play a predominant role in the energy differences between reactants and products. The scalar relativistic energy, when added to G3 theory and the empirical parameters are reoptimized, gives slightly worse agreement with experiment. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1374928
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  • 9
    Electronic Resource
    Electronic Resource
    Semiempirical models for image electrostatics. I. Bare external charge (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 2062-2066 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We develop a simple semiempirical Pariser–Parr–Pople-type model of a metal. It can be used for studies of adsorption and of the interactions between the metal surface and external charges, and is compatible with ab initio description of the molecules outside the metal. We investigate the dependence of the image-type field on the few parameters that control the model behavior, and develop a simple direct minimization self-consistent field technique that is useful for convergence in nearly degenerate cases. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1336540
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  • 10
    Electronic Resource
    Electronic Resource
    Theoretical investigation of Li and Be atom complexes with H2O (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 4230-4235 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structures, energetics, and vibrational frequencies of the Li–OH2 and Be–OH2 complexes have been examined by ab initio molecular orbital methods including the effects of electron correlation by Møller–Plesset perturbation theory. Previous studies, at the Hartree–Fock level, indicated the existence of only a very weak van der Waals-type complex between Be and H2O, while the Li–OH2 complex was strongly bound (10–11 kcal/mol) at this level. In contrast, we find that, when correlation effects are included, the Be interaction with H2O is quite similar to that of Li with H2O. The Be–OH2 complex is predicted to be nonplanar and significantly bound with a short Be–O distance of 1.736 A(ring) and a dissociation energy of 5.53 kcal/mol. The Li–OH2 complex at the correlation energy level is nonplanar with a Li–O bond distance of 1.90 A(ring) and a dissociation energy of 13.77 kcal/mol. Calculated shifts in the vibrational frequencies of H2O in Li–OH2 at the correlated level are in good agreement with the experimental values, whereas Hartree–Fock level frequency shifts are in disagreement.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.448838
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