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  • 1
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1520-510X
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 5976-5986 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The implementation of the analytical energy gradient method for the calculation of first order one electron properties using closed shell configuration interaction wave functions of the single reference plus single and double substitutions (CI-SD) type is discussed. The method used is based on the Z vector formalism of Handy and Schaefer which is readily extended to allow a full orbital optimization to be carried out for a given CI-SD wave function. The results of comparative test calculations are reported for the HF, H2O, CO, HCN, and O3 molecules, for which the dipole and quadrupole moments and the electric field gradients at the nuclei have been calculated by both the expectation value and the energy derivative formalisms using several standard basis sets. The effects of orbital optimization on the above properties at the equilibrium geometries as well as at a range of distorted geometries for HF, are also discussed. It is found that agreement between the different formalisms is best when the reference state in the CI expansion is strongly dominant. The DIIS scheme of Pulay has been incorporated into the orbital optimization method and has been found to be efficient in generating the fully optimized CI-SD/MCSCF wave functions.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 2002-2013 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using ab initio SCF molecular orbital techniques, the electric field gradients (efg's) at the oxygen and hydrogen nuclei were calculated for water clusters ranging from dimer to pentamer in an attempt to reproduce the shift in 17O and 2H nuclear quadrupole coupling constants (qcc's) that is observed on going from ice to vapor. For 2H, where the qcc shift is due mostly to the change in O–H bond length, excellent agreement with the experimental vapor → ice shift was obtained. For 17O, the change in the qcc is found to be mainly electronic in origin, effectively due to the polarization of the charge associated with the oxygen atom, and approximately 75% of the observed qcc shift was reproduced. On the basis of the calculations, estimates of the 17O and 2H qcc's in liquid water were made that are consistent with the values obtained from an analysis of the available NMR relaxation data, provided that librational motions are properly taken into account. We also present results of SCF calculations on water interacting with a Li+, Na+, or Cl− ion, indicating that the effect of a nearby ion on the 2H and 17O qcc's is similar to that produced by H bonding.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 535-544 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ab initio electronic structure calculations at the SCF and coupled pair functional (CPF) level have been carried out for the hydrogen- and "anti''-hydrogen-bonded isomers of the ClF/HF and Cl2/HF complexes. The stability of each isomer with respect to its constituent monomers has been calculated at the appropriate optimized geometry, paying particular attention to correlation effects, zero point energies, and basis set superposition errors. The differences between the latter quantities for a given pair of isomers, all calculated to be (approximately-less-than)0.5 kcal mol−1, are comparable in magnitude to the difference in stabilities, hence in a quantitative description of bonding in these dimers all these effects must be considered. The dipole moments and the 35Cl and 2H nuclear quadrupole coupling constants for the dimers have also been evaluated and compared with the available experimental data. In agreement with the results of recent molecular beam measurements, in both systems the anti-hydrogen-bonded isomers are found to be the more stable ones.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The electric field gradients (efg's) at the oxygen and hydrogen nuclei in CO,NO+, OH−, H2O, and CH2O, and at the chlorine, lithium, and hydrogen nuclei in HCl, LiCl, and FCl, calculated using ab initio quantum chemical methods, are reported. Using extended Gaussian basis sets, the efg's at the oxygen and chlorine nuclei were computed at the self-consistent field (SCF), singles and doubles configuration interaction [CI(SD)], and coupled pair functional (CPF) levels of theory as the expectation values of the efg operator and also as the energy derivatives of the appropriate perturbed Hamiltonian using the finite field method. The efg's at the hydrogen and lithium nuclei were computed as expectation values. Corrections due to zero point vibrational motions were also calculated. The effect of basis set incompleteness on the calculated efg's is discussed and, where possible, corrected for. The calculated efg's, together with the experimental nuclear quadrupole coupling constants, are used to estimate the 17O, 35Cl, and 2H nuclear quadrupole moments, and to test the quality of the correlated wave functions generated by the CI(SD) and CPF methods. The recommended values on the basis of the present calculations are −2.64±0.03, −8.2±0.2, and 0.278 fm2, respectively, for the 17O, 35Cl, and 2H nuclear quadrupole moments.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Electric field gradients (efg's) at the nitrogen nuclei in N2, NO+, NO, CN, and CN− and at the nitrogen and hydrogen nuclei in HCN, HNC, and NH3, calculated using ab initio quantum chemical methods, are reported. Employing extensive Gaussian basis sets, the efg's were computed at the self-consistent field (SCF), singles and doubles configuration interaction [CI(SD)], and coupled pair functional (CPF) levels of theory as the expectation values of the efg operator and also as the energy derivatives of the appropriate perturbed Hamiltonian using the finite field method. Corrections due to zero point vibrational motions were also calculated. The effect of basis set incompleteness on the calculated efg's, together with the experimental nuclear quadrupole coupling constants, are used to estimate the 14N and 2H nuclear quadrupole moments, and to test the quality of the correlated wave functions generated by the CI(SD) and CPF methods. The recommended values, on the basis of the present calculations, are 2.05±0.02 and 0.29±0.01 fm2, respectively, for the 14N and 2H quadrupole moments.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 5715-5727 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using ab initio Hartree–Fock SCF, configuration interaction, and approximate coupled pair techniques, an extensive quantum chemical study of the infrared absorption intensities of the water molecule is reported. In an attempt to establish the necessary requirements for the quantitative prediction of infrared intensities four major effects were examined: (a) the magnitude of electron correlation effects, (b) the applicability of the Hellmann–Feynman theorem to the calculation of dipole moments functions, (c) basis set requirements, and (d) the accuracy of the customary harmonic approximations. The calculated infrared intensities for the water molecule and several of its isotopic derivatives are in good agreement with the available experimental data. In addition to the fundamentals the intensities of several overtone and combination bands were also calculated resulting in generally very small values that strongly depend on the potential energy surface used in the generation of vibrational wave functions.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 5721-5730 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using ab initio SCF, singles and doubles configuration interaction (CI-SD), and coupled pair functional (CPF) techniques with basis sets of triple zeta plus two polarization functions quality, the following properties have been computed for sulfur dioxide at its equilibrium geometry: dipole and quadrupole moments, electric field gradients at the nuclei, static and frequency dependent dipole polarizability (at λ=5154 A(ring) and λ=6328 A(ring)), dipole and polarizability derivatives, infrared absorption intensities, and Raman scattering activities. The properties, when calculated at the correlated level of theory have been evaluated by the energy derivative (ED) as well as the expectation value (EV) formalism. The results of the CPF(ED) calculations are in good agreement with the available experimental data and on the basis of additional two-configurational MCSCF studies the use of single reference CI-SD and CPF wave functions is judged to be justified. The analogous calculations on ozone serve to highlight the considerable differences between the two molecules, inasmuch as due to the considerably larger degree of near-degeneracy in O3, properties that involve mixing of the lowest 1B2 excited state with the 1A1 ground state are predicted poorly at the SCF level as well as at the single reference CI-SD and CPF levels of theory.
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 1520-510X
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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