BRILLOUIN-WIGNER BASED MULTIREFERENCE PERTURBATION-THEORY FOR ELECTRONIC CORRELATION-EFFECTS

We investigate basis set reduction (BSR), a configuration-based multi- reference perturbation theory using an implicit effective Hamiltonian motivated by Brillouin-Wigner perturbation theory. This approach avoid s the intruder-state and level-crossing problems by construction and y ields a rapidly converging perturbation expansion. Formulated to syste matically approximate multi-reference configuration interaction, BSR y ields accurate results in second order, because it includes relaxation effects of the primary space wavefunction in the presence of the pert urbation. We benchmark the method for molecules (CH2,O-3) in which bot h dynamical and non-dynamical correlation effects are known to be impo rtant, obtaining accuracies of the order of 1 kcal/mol across the pote ntial energy surface in second-order perturbation theory. We address t he critical issues of perturbative orbital optimization for the primar y orbital space, the choice of the secondary orbital space and the eff ects of single excitations. (C) 1998 American Institute of Physics.