Construction of model hamiltonians in framework of Rayleigh-Schrödinger perturbation theory

A theory of the model Hamiltonians within the framework of Rayleigh-Schrödinger perturbation theory is elaborated. The approach of a model Hamiltonian is based on the assumption that if it is diagonalized in a chosen model space it will yield eigenvalues of the original Hamiltonian in the entire Hilbert space. The theory of the model Hamiltonians may be fruitful as a theoretical background for the study of effective Hamiltonians and as natural extension of the standard Rayleigh-Schrödinger perturbation theory.