A non-perturbative open-shell theory for ionisation potential and excitation energies using HF ground state as the vacuum

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Abstract
The authors' recently developed (1975) non-perturbative open-shell theory is adapted for direct evaluation of ionisation potential and excitation energies. Separating the 'core' contribution from the 'valence' part through the use of a multiple-cluster-expansion operator, the method provides a systematic way of including various core-valence interaction and core relaxation components. HF orbitals have been used as the basis for facilitating detailed comparison of the method with perturbative and propagator techniques. It has been shown that the method encompasses all the important second-order contributions of the latter formalisms and certain other classes of diagrams in a compact manner. An application to a simple 4 pi electronic problem is discussed to illustrate how this method works for real systems.