Perturbation Theory and Effective Hamiltonian

Abstract

Perturbation theory is one of the standard subjects in elementary quantum mechanics. The connection to renormalization emerges naturally if one adopts a generalized view on perturbation theory. While the ordinary perturbation theory derives approximate eigenenergy and the corresponding eigenfunction, the generalized view derives an operator called the effective Hamiltonian. In this chapter we reformulate the perturbation theory by introducing the concept of model space. As a simple application of the effective Hamiltonian, we discuss the spin structure of hydrogen and oxygen molecules.