Alternative derivation of perturbation expansions in the projection operator formalism

Abstract

AbstractThe perturbation‐theoretic expansions obtained from Löwdin's projection operator formalism are derived in a new way, using Kato's formulation of perturbation theory. Kato's approach provides a convenient alternative to diagrammatic techniques for obtaining eigenvalues and eigenvectors. Different normalization criteria imposable on the wave function are easily visualized in terms of the operator that yields the perturbed state vector when it acts upon the unperturbed wave function.