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Second-order perturbation theory using correlated orbitals. II. A coupled MCSCF perturbation strategy for electronic spectra and its applications to ethylene, formaldehyde and vinylidene

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Published in Chemical Physics. 1996, vol. 205, no. 1, p. 323-349
Abstract In this second paper, the philosophy of coupling multiconfigurational variational wave functions to perturbation treatments (MC/P methodology) is extended to the calculation of electronic spectra. The corresponding methodology is presented with emphasis on its flexibility and an overview of other available approaches is given. The contracted MC/P scheme is then applied to ethylene H2C=CH2, formaldehyde H2C=O vinylidene H2C=C. It is shown that combining well-designed averaged zeroth-order MCSCF wave functions to a barycentric Møller-Plesset (BMP) partition of the electronic Hamiltonian provides accurate spectra, contrary to Epstein-Nesbet partitions. The MC/BMP transition energies compare with experimental data within a few hundreds of cm−1. These results have been obtained using a polarized double-zeta quality basis set augmented by a set of semi-diffuse functions (6–31 + G*) and by an extra set of diffuse orbitals to account for Rydberg states. Since non-dynamic correlations effects that are important for a proper description of the manifold of the excited states of interest are included in the MCSCF zeroth-order space will all remaining correlation effects (non-dynamic and dynamic) are treated at the perturbation level, the present study lets anticipate applications of the MC/P methodology to medium size systems without much computational trouble.
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Research group Groupe Weber
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PARISEL, Olivier, ELLINGER, Yves. Second-order perturbation theory using correlated orbitals. II. A coupled MCSCF perturbation strategy for electronic spectra and its applications to ethylene, formaldehyde and vinylidene. In: Chemical Physics, 1996, vol. 205, n° 1, p. 323-349. https://archive-ouverte.unige.ch/unige:2820

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