Abstract
The non-degenerate finite-order many-body perturbation theory is developed with respect to a bare-nucleus reference function. The state consistency and size consistency of this approach to the calculation of molecular electronic structure is emphasised. The bare-nucleus many-body perturbation theory is applied to three simple model systems involving four hydrogen atoms in various spatial arrangements. The results are compared with those obtained by many-body perturbation theory developed with respect to a self-consistent-field wavefunction and full configuration interaction. The effects of quasidegeneracy in the three model systems on the convergence of the perturbation expansion are investigated.
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