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  • Digitale Medien  (8)
  • 1985-1989  (8)
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Materialart
  • Digitale Medien  (8)
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  • 1
    Digitale Medien
    Digitale Medien
    Spin density of radicals by finite field many-body methodsf a@f ) (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 4225-4229 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The spin densities of several small radicals CH3, NH2,CH2CH, BeH, and H2CO+ are calculated by a finite field coupled-cluster method using a spin density operator as a finite perturbation on the systems. The calculated hyperfine coupling constants of the π radicals are in good agreement with experiment at a low level of correlation. The CH2CH radical required a higher level of correlation to obtain the experimental value. The calculations are also performed for the H2CO+ radical where theory has failed to predict the experimental splitting constants. The spin density on hydrogen calculated in the present study is lower than the result obtained by the low temperature gas ESR techniques, although the carbon spin density is well reproduced. An analysis of possible corrections for the H spin density is presented.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.448837
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  • 2
    Digitale Medien
    Digitale Medien
    Nuclear spin–spin coupling constants evaluated using many body methods (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 3945-3949 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: In nuclear spin–spin coupling constant determinations, correlation corrections to the Fermi contact term are significant. In this paper we report the coupling constants calculated for the HD and HF molecules obtained by the infinite-order coupled cluster singles and doubles (CCSD) methods and MBPT(4). These are in good agreement with the experimentally estimated value for the Fermi-contact term. In addition, it is well known that the coupled perturbed Hartree–Fock (CPHF) scheme fails for multiply bonded molecules because the closed shell Hartree–Fock solution is triplet unstable. A CCSD method using ordinary nonrelaxed SCF orbitals is presented in order to circumvent this problem, and illustrated by application to the C2H4 molecule. It is shown that CCSD results based upon ordinary SCF orbitals include effectively all the effect of orbital relaxation and reproduce the experimental values for most of the coupling constants. Unlike previous results, the 3J(H–H) constant is positive in agreement with experiment.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.450916
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  • 3
    Digitale Medien
    Digitale Medien
    Property evaluation and orbital relaxation in coupled cluster methods (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 502-509 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Molecular electronic properties such as dipole moments, polarizabilities and hyperpolarizabilities and quadrupole moments and polarizabilities, and spin properties such as hyperfine splitting constants and nuclear magnetic coupling constants are predicted by ab initio coupled cluster (CC) methods for a variety of molecules. We compare the results of property evaluation using orbitals that have been allowed to relax in the presence of the perturbation and results obtained using nonrelaxed orbitals. It is demonstrated numerically, and proven formally, that the coupled cluster singles and doubles (CCSD) model using nonrelaxed orbitals is able to include most of the relaxation effects for the evaluation of first- and second-order properties. Thus there is little reason to perform coupled (perturbed) Hartree–Fock calculations as a precursor to correlated CCSD calculations of such properties.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.453596
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  • 4
    Digitale Medien
    Digitale Medien
    A screened potential molecular-orbital calculation of the π-electron systems of metalloporphin, metallochlorin, and metallobacteriochlorin (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 5045-5052 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The excitation energies and oscillator strengths of the lowest (π,π*) excited states of metalloporphin, -chlorin, and -bacteriochlorin were calculated by the screened potential molecular-orbital theory. The screened potential incorporates instantaneous polarization effects in the molecular Hartree–Fock field. The model reproduced the spectra of the lowest excited states fairly well without use of empirical parameters. The calculated energies of the lowest excited triplets and the Q singlet state were in good agreement with those observed, while the B state was correctly predicted only for the reduced resonance integrals.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.452681
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  • 5
    Digitale Medien
    Digitale Medien
    Hyperpolarizabilities of the hydrogen fluoride molecule: A discrepancy between theory and experiment? (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 2726-2733 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Due to the recent availability of an experimental value for the second- and third-order electric polarizabilities of the HF molecule, possible errors in the previous prediction of these quantities have been considered. These include basis sets, vibrational corrections, frequency dependence, infinite-order correlation corrections, and the effect of triple excitations. Despite the inclusion of all of these effects, the discrepancy between experiment and theory remains. Our best results predict χ(2)(parallel) to be −3.3 to −3.8×10−32 and χ(3)(parallel) to be 45 to 48×10−39 esu which are at best 81% and 69% of the experimental values, respectively. Possible additional corrections are considered with emphasis on basis set completeness. Considering the difficulty in predicting such sensitive quantities, this is rather good agreement. The paper also addresses questions of reliability of ab initio calculations for such high-order properties as hyperpolarizabilities, while identifying some places where the experimental results are susceptible to error.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.450348
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  • 6
    Digitale Medien
    Digitale Medien
    Frequency dependent nonlinear optical properties of molecules (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 976-989 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Various nonlinear optical polarizabilities are derived and evaluated by time dependent Hartree–Fock theory (TDHF). The recursive nature of the TDHF theory is exploited to develop formulas that are applicable in any order. The theory is applied to evaluate dispersion effects for the series of molecules CH4, CH3F, CH2F2, CHF3, and CF4. Comparisons are made with results obtained from dc-induced, second-Harmonic generation, and third-Harmonic generation experiments. Additional applications are reported for H2 and HF.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.451255
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  • 7
    Digitale Medien
    Digitale Medien
    On the use of gaussian-type functions in dirac-fock basis set expansion calculations (1988)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 34 (1988), S. 457-463 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Dirac-Fock balanced Gaussian basis calculations that employ an extended nucleus model (see Ref. 11) have been performed on Be and Ne atoms by systematically enlarging the basis set. As expansion size increases, the total energy smoothly converges toward the numerical Dirac-Fock limit from above. The balanced expansion method does not introduce any of the variational instabilities that have plagued the early Dirac-Fock basis expansion calculations.
    Zusätzliches Material: 2 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560340849
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  • 8
    Digitale Medien
    Digitale Medien
    Coupled-cluster evaluation of geometrical derivatives of properties using nonrelaxed orbitals (1987)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 32 (1987), S. 487-493 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Analytical derivatives are dependent upon derivatives of the Ao basis set and the MO relaxation. By separating these effects on derivatives, they can be individually studied. We show that the coupled-cluster single and double (CCSD) model introduces most of the MO relaxation effects required by gradients, hessians, dipole, and polarizability derivatives. This finding should make it possible to simplify analytical derivative evaluation at the CCSD level.
    Zusätzliches Material: 2 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560320747
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