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  • 1
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A multireference Møller–Plesset (MRMP) method with a complete active space configuration interaction (CASCI) reference function has been proposed as an accurate and computationally efficient method for treating the ground and excited states of molecules. The CASCI wave function was constructed using the self-consistent field orbitals and used as a reference function of the MRMP to incorporate the remaining dynamical correlation. The advantage in using the CASCI is that it does not require iterations, nor does it encounter convergence difficulties which may be found in complete active space self-consistent field (CASSCF) calculations. The scheme was applied to the potential curves of the ground and low-lying excited states of N2, the potential curve of the ground state of CO, and the vertical valence π–π* excited states of benzene. Excellent agreement between theory, experiment, and some benchmark calculations was obtained. MRMP with CASCI is comparable in accuracy with MRMP with CASSCF. The present scheme will open a possibility toward accurate treatment for the ground and excited states of small to large molecular systems. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7773-7778 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Excited states of C2, H2O, CO, and formamide are studied to illustrate the performance and accuracy of second-order diagrammatic complete active space perturbation theory (D-CASPT2) [J. Chem. Phys. 108, 1081 (1998)]. Comparisons are made with other ab initio methods and also full configuration interaction (FCI) calculations or experiment. Excitation energies computed by the D-CASPT2 method are quite accurate showing an average deviation of 0.1 eV from the FCI values for C2 and H2O. The CO and formamide excitation energies yield average deviations of 0.1 and 0.2 eV from experiment, respectively. The computational cost of this method is reduced to a great extent compared to the MRMP method. © 2000 American Institute of Physics.
    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 111 (1999), S. 3837-3845 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ten low-lying electronic states of Fe(II) porphine, 5A1g, 5Eg, 5B2g, 3A2g, 3B2g, 3Eg(A), 3Eg(B), 1A1g, 1B2g, and 1Eg states, are studied with multiconfigurational second-order perturbation (CASPT2) calculations with complete active space self-consistent field (CASSCF) reference functions with larger active space and basis sets. The enlargement of active space and basis sets has no influence on the conclusion of a previous multireference Møller–Plesset perturbation (MRMP) study. The present CASPT2 calculation concludes that the 5A1g state is the ground state. A relativistic correction has been performed by the relativistic scheme of eliminating small components (RESC). For energetics, no significant contribution from the relativistic correction was found. The relative energies and orbital energies are not changed appreciably by the introduction of a relativistic correction. The present result does not agree with all the spectroscopic observations, but is consistent with a magnetic moment study. © 1999 American Institute of Physics.
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  • 4
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The lowest-lying excited states of N2 and CO are examined for the presence of intruder states that adversely influence the perturbative convergence in computations using multireference Møller–Plesset perturbation theory. Intruder states are found in the computations of the 3Πu state of N2 and the 3Π state of CO. Since, in second order, these intruders—under normal circumstances—should make a negligible contribution to the target state, their contributions to the second-order energies are simply removed from the perturbation expansion. With the intruder state removed, the vertical excitation for the 3Πu (3Π) state of N2 (CO) is only 0.23 eV (0.35 eV) below the experimental value, compared to differing by 0.72 eV (0.44 eV) with the intruder state present. The intruder states are also removed by performing energy-denominator shifts using a redefined zeroth-order Hamiltonian H0 that differs from previous treatments, since only the energy denominator associated with the intruder state is shifted. This approach removes intruders, but makes no other changes to the perturbation expansions. Two-state models are used to identify the intruder states and suggest remedies. © 2001 American Institute of Physics.
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