Use of biorthogonal orbitals in perturbation calculation of molecular interactions from a multiconfigurational unperturbed state. II

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Abstract
It has been shown in a previous paper that molecular interactions can be diagrammatically expanded using biorthogonal orbitals and non‐Hermitian operators so that all intramolecular terms cancel. The purpose of this paper is to extend this method to the general case where degenerate and quasidegenerate configurations appear both in each subsystem and in the supersystem. We also study in more detail the development of the wavefunctions and the consequences on varied observables.