On the use of spin graphs for spin adapting many‐body perturbation theory

Abstract

AbstractIn connection with spin adaptation in many‐body perturbation theory, this paper reexamines the use of spin graphs in view of a Hugenholtz‐like representation where both the orbital and the spin parts coexist. Together with the idea of essentially distinct diagrams, this representation leads to an economic handling of spin adaptation. As a side issue, the appropriate generalization of the Epstein–Nesbet partitioning for spin‐adapted perturbation theory is obtained. Compact formulas up to fourth order of the ground‐state energy, and up to third order for excitation energies and ionization potentials are given.