On the choice of the zero-order operator in perturbation theory

Abstract

Given a basis, the matrix representation of a hermitian operator Ô = Ô (o)+ô (1) is partitioned ω=ω (o)'+ω (1)' such that ω (o) and ω (o)' have the same eigenvectors and the euclidean norm of ω (1)' is a minimum. This splitting corresponds to the so-called Epstein-Nesbet partition in perturbation theory.