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  • 1
    Electronic Resource
    Electronic Resource
    The equilibrium structure and harmonic vibrational frequencies of ozone: Coupled cluster results including triple excitations (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1945-1947 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The highly correlated CCSDT-2 model of electron correlation is applied to computation of the equilibrium geometry and harmonic vibrational frequencies of O3. Comparison of the present results with those of a recent study [J. F. Stanton, W. N. Lipscomb, D. H. Magers, and R. J. Bartlett, J. Chem. Phys. 90, 1077 (1989)] reveal the importance of certain simultaneous three-particle correlation effects (T3) which enter perturbation theory in fifth and higher orders, particularly for asymmetric structures of ozone and, consequently, the B2 stretching frequency. The CCSDT-2 value for ω3 is 1182 cm−1, in reasonable agreement with the experimental value of 1089 cm−1, and considerably better than values obtained with other methods which make more approximations in treating T3. To treat this difficult problem, it appears to be necessary to include terms involving T3 which are nonlinear in the cluster amplitudes into the CC equations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457053
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  • 2
    Electronic Resource
    Electronic Resource
    Correlated studies of infrared intensities (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 3241-3249 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Coupled-cluster and many-body perturbation theories are applied to an investigation of infrared absorption intensities within the double-harmonic approximation. In agreement with previous studies, both electron correlation and basis set dependencies are found to be significant, particularly for stretching vibrations which involve hydrogen atoms. Intensities calculated at the highly correlated CCSD+T(CCSD) level with large Gaussian basis sets are in reasonable agreement with experiment for HF and the ν2 and ν3 modes of water, while the intensity of ν1 is significantly overestimated even with a relatively large 53-CGTO basis. In addition, intensities and harmonic frequencies calculated at the SCF and MBPT(2) levels with a double-zeta plus polarization (DZP) basis set are presented for a number of first row compounds, and are compared to recent experimental values. Although agreement between experimental and SCF intensities is poor, these discrepancies are moderated considerably when correlation is introduced at this simple level. The importance of including hydrogen polarization functions in the basis set is also discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455876
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  • 3
    Electronic Resource
    Electronic Resource
    Highly correlated single-reference studies of the O3 potential surface. I. Effects of high order excitations on the equilibrium structure and harmonic force field of ozone (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 1077-1082 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The potential energy surface of ozone in the vicinity of the equilibrium geometry is investigated by single-reference many-body perturbation theory (MBPT) and coupled-cluster (CC) methods. As expected from the known inadequacies of the independent-particle picture of O3, analysis of the CCSDT-1 wave function reveals considerable mixing between the [core⋅⋅⋅]4b226a211a22 and [core⋅⋅⋅]4b226a212b21 configurations. Smaller, but still significant, contributions come from other configurations involving redistribution of electrons within the out-of-plane π orbital framework. As expected, the equilibrium structure and harmonic force field computed at the SCF level of theory are in considerable error. When allowance is made for electron correlation effects, the discrepancies between theory and experiment for the equilibrium structure and totally symmetric force field are significantly reduced, and the MBPT(4), CCSD, CCSD+T(CCSD) and CCSDT-1 results are in reasonable agreement with accepted values. Asymmetric stretching frequencies, however, are found to oscillate wildly [2373 cm−1 at MBPT(2), 1547 cm−1 at MBPT(4), and 680 cm−1 at CCSDT-1] about the experimental value of 1089 cm−1, suggesting that the Cs subspace of the potential surface poses a great challenge to theory. A possible cause of this behavior is discussed, along with implications for future studies of the global O3 potential surface.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456161
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  • 4
    Electronic Resource
    Electronic Resource
    Structure, energetics, and vibrational spectra of beryllium borohydride isomers (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 5726-5734 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Several structures of beryllium borohydride (BeB2H8) have been studied with techniques based on many-body perturbation theory (MBPT). Energies calculated at the MBPT(4) level with a basis set of 85 contracted Gaussian functions support the two structure hypothesis of Nibler, although a qualitative MBPT(4) asymmetric stretching potential indicates that the C3v double-minimum structure is unstable with respect to a more highly symmetric D3d model with two sets of equivalent triple hydrogen bridges. The theoretical free energy difference between the D3d structure and a diborane-like model having D2d symmetry is 1.1 kcal/mol, with the latter predicted to be more stable. Uncertainties in the theoretical force field and those resulting from the finite nature of the basis set, however, may change this value by several kcal/mol. Principal evidence for the two-structure proposal comes from MBPT(2) harmonic frequencies and infrared absorption intensities, which are found to be consistent with the published vibrational spectra. These are used in a tentative assignment of most of the observed features. Unfortunately, the experimentally observed dipole moment of 2.06 D cannot be explained at present.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454532
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  • 5
    Electronic Resource
    Electronic Resource
    A diagrammatic construction of a Rayleigh–Schrödinger multiple perturbation theory (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 5182-5191 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A Rayleigh–Schrödinger multiple perturbation theory is presented in a general formulation useful for energy evaluations beyond the Hartree–Fock level. This theory allows one to computationally take advantage of the structure of the basis set. For example, three- and four-center-like integrals appearing in the second quantized form of the Hamiltonian may be treated as higher order perturbations. Explicit formulas for the wave function to second order and energy to fourth order are also diagrammatically derived. As examples the theory is applied to aromatic cyclic polyenes within the PPP approximation. A perturbation sequence is defined by the magnitude of the integrals. Within this model errors of less than 1 kcal/mol can be obtained even while neglecting 50% of the integrals up to fourth order. Preliminary studies with ab initio calculations show that even a greater percentage of two electron integrals can be neglected with little loss of accuracy when using this scheme.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449730
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