The Rayleigh-Schrodinger perturbation and the linked-diagram theorem for a multi-configurational model space

The Rayleigh-Schrodinger perturbation formalism is extended to the case of a model space, which is not necessarily degenerate. The model space defines the zero-order or model wavefunction, and the new formalism makes it possible to use a model wavefunction of multi-configurational type. The effect of the states outside the model space are taken into account by means of a perturbation expansion and expressed in terms of an 'effective' Hamiltonian, operating only within the model space. The extended Rayleigh Schrodinger formalism is used to prove the linked-diagram theorem for a multi-configurational model space in a simple way. The problem of convergence of the perturbation expansion is briefly discussed.