Use of Multiple Basis Sets in the Brueckner-Goldstone Many-Body Perturbation Theory for Atomic Problems

Abstract

In the Brueckner-Goldstone many-body perturbation theory as applied to atomic calculations, the origin of important higher-order diagrams is associated with the inflexibility of the single basis set upon which the perturbation expansion is normally based. We show that in calculating each diagram one may employ more than one basis set simultaneously. The use of multiple basis sets is identical to the exact inclusion of a large class of important higher-order terms. We give illustrated examples to show where this technique will be of practical importance. This possibility is expected to improve the accuracy and to extend the range of applicability of the Brueckner-Goldstone many-body perturbation-theory approach.