2ND-ORDER PERTURBATION-THEORY USING CORRELATED ORBITALS .1. FULL-VALENCE REFERENCE FUNCTIONS

Recent developments of performant perturbation treatments on multiconf iguration wave functions have renewed interest in the coupling of vari ational and perturbative methods. In this communication it is shown th at the choice of both the orbitals and the perturbation Hamiltonian to be used is as crucial as the choice of the reference space for obtain ing accurate results. Moller-Plesset and Epstein-Nesbet perturbation s eries are applied to full-valence configuration interaction (FVCI) wav e functions built on MCSCF (multi-configurational self-consistent held ), FOCI (first-order configuration interaction) and SOCI (second-order configuration interaction) natural orbitals. Applications are present ed for the following well-known systems: CH2 (X(3)B(1)-a(1)A(1)), CH2 (X(2)A(1), 1(2)B(1), 1(2)A(2), 1(2)B(2)), SiH2 (X(1)A(1), a B-3(1), A (1)B(1)) and NH2 (X(2)A(1), A(2)B(1)). The results are compared to the corresponding full configuration interaction (FCI) when available.