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  • 1
    Digitale Medien
    Digitale Medien
    Alkali oxides. Analysis of bonding and explanation of the reversal in ordering of the 2.PI. and 2.SIGMA. states (1984)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 88 (1984), S. 1262-1268 
    Details
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie , Physik
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1021/j150650a049
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  • 2
    Digitale Medien
    Digitale Medien
    A theoretical investigation of the ground and low-lying excited states of butadiene radical cation (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 3745-3755 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Results are presented from ab initio calculations on the ground and several low-lying excited states of the butadiene radical cation. In particular, we have calculated geometries for the ground and several excited states at the multiconfiguration self-consistent field level and characterized the planar stationary points. The vertical ionization potentials from the neutral molecule ground state and vertical excitation energies at the computed equilibrium geometry of the cation were also examined. A variety of methods were tested for the calculation of ionization potentials and excitation energies, including multiconfiguration self-consistent field, multireference singles and doubles configuration interaction (with and without size-consistency correction), and multireference averaged coupled pair functional theory. It is found that several of the excited states are strong mixtures of so-called Koopmans' and non-Koopmans' configurations. In most cases, good agreement is found with experiment, but the excitation energy for the second excited π state, a state which is multiconfigurational at zeroth order, is apparently overestimated at all levels of theory employed here.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.461879
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  • 3
    Digitale Medien
    Digitale Medien
    Quasidegenerate variational perturbation theory and the calculation of first-order properties from variational perturbation theory wave functions (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 6798-6814 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: In previous work on the treatment of correlation in molecular systems we have applied a multireference version of second-order Hylleraas variational perturbation theory. The choice made for the partitioning of H treated the interactions between the correlating functions to infinite order and gave the corrections to the wave function to first order. The method was shown to be accurate in many cases, but became less so when near degeneracies occurred between the reference energy and other eigenvalues of H0. In this article we introduce an effective Hamiltonian method that is analogous to variational perturbation theory, but which is significantly more accurate when near degeneracies are important. This quasidegenerate variational perturbation theory (QDVPT) is an explicitly multireference procedure and treats the entire reference space as a quasidegenerate space. A novel method for solving the QDVPT equations is introduced that avoids explicit construction of the effective Hamiltonian. As a result, the work involved in application of QDVPT is on the roder of that required for variational perturbation theory. We also present an approximate method for calculating first-order atomic and molecular properties based on Hylleraas variational perturbation theory, multireference linearized coupled cluster, and QDVPT wave functions. The properties are calculated as derivatives of the energy with respect to the field strength. Construction of a one-electron density matrix based on the energy derivative expression allows rapid evaluation of one-electron properties. Results are presented and compared to full and truncated CI results. Good agreement is found in the cases examined.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.455354
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  • 4
    Digitale Medien
    Digitale Medien
    Size-inconsistency effects in molecular properties for states with valence-Rydberg mixing: The low-lying π→π* states of ethylene and butadiene (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 2450-2456 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Ab initio results for the low-lying 1B1u states of ethylene are used to discuss size-inconsistency effects on calculated molecular properties for states where valence-Rydberg mixing is important. Results for the expectation value of x2, x being the coordinate perpendicular to the plane of the molecule, are presented from quasidegenerate variational perturbation theory and multireference configuration-interaction calculations. These results are compared with values from previous studies. It is argued that size inconsistency in configuration-interaction studies can have a significant effect on estimated molecular properties. Calculations on several low-lying states of butadiene are also reported where similar size-inconsistency effects are found.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.457987
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  • 5
    Digitale Medien
    Digitale Medien
    Hylleraas variational perturbation theory: Application to correlation problems in molecular systems (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 5770-5778 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Hylleraas variational perturbation theory is applied through second order in energy to estimate the correlation energy in several molecular systems. The specific choices for H0 and V which are made lead to equations nearly identical to the multireference linearized coupled-cluster method of Laidig and Bartlett. The results obtained are in virtually exact agreement where comparisons have been made. Results from test calculations are presented for BeH2, CH2, and C2H4. In addition, the utility of perturbation theory for selecting correlating configurations is examined. This procedure is found to be quite accurate while significantly reducing the size of the system of linear equations to be solved.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.454535
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  • 6
    Digitale Medien
    Digitale Medien
    A theoretical investigation of the geometries, vibrational frequencies, and binding energies of several mixed alkali halide dimers (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 9764-9769 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Results are presented from ab initio calculations on several mixed alkali halide dimers made up of Li, Na, F, and Cl. All of the dimers are investigated at the restricted Hartree–Fock level to provide ab initio estimates of geometries, vibrational frequencies, and binding energies. The dimer LiNaF2 is then treated using a variety of basis sets at the Hartree–Fock level, as well as at correlated levels (second-order Møller–Plesset and coupled-cluster singles and doubles) to examine the sensitivity of the calculations to use of higher levels of theory. The results are then compared to the experimental data available for the LiNaF2 molecule, previous theoretical results for the monomers, and recent semiempirical calculations on the mixed dimers.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.465458
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  • 7
    Digitale Medien
    Digitale Medien
    A theoretical investigation of the geometries, vibrational frequencies, and binding energies of several alkali halide dimers (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 2182-2190 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Results are presented from ab initio calculations on the symmetrical alkali halide dimers made up of Li, Na, K, F, and Cl. We examine the sensitivity of representative monomer and dimer geometries to the variation of the basis set with and without polarization and diffuse functions. The geometries are then compared with available experimental results. We have also calculated vibrational frequencies at the restricted Hartree–Fock level and examined the changes in geometry brought about by correlation using second-order Møller–Plesset perturbation theory. It is found that Hartree–Fock theory in a modest basis set with diffuse and polarization functions yields results comparable to much larger sp basis sets and that the theoretical results are in good agreement with the experimental results for the Li and Na dimers. Our best results for the K-containing species tend to have bond lengths that are too long for the monomers and this error is carried over for the dimers. We also find a nearly uniform expansion of the M–X bond length in proceeding from monomer to dimer of 0.16±0.03 A(ring), independent of the alkali or halide involved. The calculated dimer dissociation energies are in excellent agreement with experiment. Inclusion of correlation appears to have a minimal effect on the computed geometries and a modest effect on the binding energies. The vibrational frequencies for the monomers are in excellent agreement with experimental gas-phase results and reasonable agreement is obtained with the available experimental frequencies for the dimers. Finally, a reanalysis of the electron diffraction data for Na2F2 in light of the current ab initio results leads to a significant change in the experimental value of the bond angle.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.464197
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  • 8
    Digitale Medien
    Digitale Medien
    A CASSCF study of various rotamers of the hexatriene radical cation (1992)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 96 (1992), S. 5332-5336 
    Details
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie , Physik
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1021/j100192a029
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  • 9
    Digitale Medien
    Digitale Medien
    Theoretical Study of Solvent Effects on the Electronic Coupling Matrix Element in Rigidly Linked Donor-Acceptor Systems (1995)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 99 (1995), S. 17501-17504 
    Details
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie , Physik
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1021/j100049a006
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  • 10
    Digitale Medien
    Digitale Medien
    Ab initio investigation of several low-lying states of all-trans octatetraene (1988)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 92 (1988), S. 2173-2177 
    Details
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie , Physik
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1021/j100319a019
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