New Time-Independent Perturbation Theory for the Multireference Problem

摘要

A new time-inependent pertubation theory is developed for the multireference problem. In the derivation, neither perturbed wave function nor intermdiate norintermediate normalization condition is required.In the single-redernce case, the present approach gives the same present approach gives the same pertubation epressions 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 thithout the definition of the zeroth Hamiltonian H_o. 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 pertubation expansions are furthr expossed with the eigenvectors of the Hamiltonian withinthe reference space for an efficient implementation. The correspondence between single-reference and multireference perturbation expansion are further expressed with the eigenvectors of the Hamiltonan within the reference space for an efficent implementation. The correspondences between the single-reference and multireference perturbation expansion are discussed