Perturbation theory of effective interactions for many-electron systems in terms of nonorthogonal basis states

Diagrammatic quasidegenerate Rayleigh-Schr\"odinger perturbation theory is used for the construction of an effective Hamiltonian, defined completely in a finite-dimensional model space. This effective Hamiltonian, containing one-, two-, three-, etc. particle terms, may serve as a useful tool for constructing an ab initio theory of quantum semiempirical effective Hamiltonians, widely used in quantum molecular and atomic physics, and also in some modified form in solid-state physics.