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  • American Institute of Physics (AIP)  (6)
  • 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
    Stability and properties of C4 isomers (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 3612-3617 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The relative electronic energies of the 1Ag rhombus and the 3Σ−g linear isomers of C4 have been computed using a 5s3p1d basis at various levels of coupled-cluster and many-body perturbation theory. At the highest level performed, CCSD+T(CCSD), the two isomers are essentially isoenergetic. Themodynamic results indicate, however, that the linear isomer will be significantly more abundant in the plasma in which C4 is formed. Vibrational frequencies are presented and compared with experimental results; some of the prior assignments are questioned. The isotropic ESR hyperfine splitting parameters for the linear triplet state have been computed and are also compared with experimental results. Finally, the ionization potential and electron affinity for both structures are presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454881
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  • 5
    Electronic Resource
    Electronic Resource
    Computation of quadratic electric dipole moment functions (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 7650-7652 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A numerically reliable method for the calculation of molecular electric dipole moment functions is described. This scheme, which is an extension of a method due to Komornicki and McIver, is applied to the computation of the quadratic dipole moment function of water using many-body perturbation theory. A pronounced basis set dependence is observed for the stretch and stretch–stretch coefficients of the quadratic expansion of the dipole moment, while those involving the bending symmetry coordinate appear to be less sensitive. In addition, previously published ab initio dipole moment functions are discussed and compared to the present results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454278
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  • 6
    Electronic Resource
    Electronic Resource
    Isomers and excitation energies of C4 (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 3284-3290 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Coupled-cluster (CC) and many-body perturbation theory (MBPT) studies of the rhombic and linear structures of C4 are reported. For each isomer, the electronic spectra is obtained, and comparisons are made with experimental matrix-isolated ESR and electronic spectra. The closed-shell 1Ag rhombic ground state is found to be more stable than the 3∑−g state of the linear isomer by 5 kcal/mol at the highest level of calculation performed (CCSDT-1). However, the predicted spectrum for linear C4 is in reasonable agreement with the observed results. An allowed electronic transition for the rhombus is predicted to lie in the same region, suggesting the possibility that both isomers could coexist in the experiment. Finally, vibrational frequencies for the rhombic isomer are calculated using analytical second-order MBPT second derivatives to aid in the experimental identification of this transient species.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450259
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