Abstract
Abstract Recent developments of performant perturbation treatments on multiconfiguration wave functions have renewed interest in the coupling of variational and perturbative methods. In this communication it is shown that the choice of both the orbitals and the perturbation Hamiltonian to be used is as crucial as the choice of the reference space for obtaining accurate results. Moller-Plesset and Epstein-Nesbet perturbation series are applied to full-valence configuration interaction (FVCI) wave functions built on MCSCF (multi-configurational self-consistent field), FOCI (first-order configuration interaction) and SOCI (second-order configuration interaction) natural orbitals. Applications are presented for the following well-known systems: CH2 (X3B1−a1A1), CH2+ (X2A1, 1 2B1, 12A2, 1 2B2), SiH2 (X1A1, a 3B1, A 1B1) and NH2 (X 2A1, A 2B1). The results are compared to the corresponding full configuration interaction (FCI) when available.
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