New time-independent perturbation theory for the multireference problem

Abstract

A new time-independent perturbation theory is developed for the multireference problem. In the derivation, neither perturbed wave function nor intermediate normalization condition is required. In the single-reference case, the present approach gives the same perturbation expressions as Rayleigh-Schrodinger perturbation theory. In the multireference case, the perturbation expressions are derived with two kinds of partitions and can be applied to those cases with or without the definition of the zeroth Hamiltonian H 0 . As the size of the reference space is decreased to 1, one multireference expansion returns to the single-reference one with Epstein-Nesbet partition, while another is reduced to a new expansion. These two multireference perturbation expansions are further expressed with the eigenvectors of the Hamiltonian within the reference space for an efficient implementation. The correspondences between the single-reference and multireference perturbation expansion are discussed.