Unitary transformation on the model space. Comparison of Brandow's and Kirtman's versions of quasidegenerate many body perturbation theory

Abstract

AbstractIn this paper we study the unitary transformations on the model space in quasidegenerate many body perturbation theory (QDMBPT). From this point of view we compare Brandow's and Kirtman's versions of QDMBPT up to the fifth order of perturbation expansion. We show that, starting from Brandow's QDMBPT, we can derive various versions of effective Hamiltonians order by order, of which the Kirtman's version is the simplest one. The operator of unitary transformation we express through the correlation operator.