Many-body perturbation theory applied to electron pair correlation energies. I. Closed-shell first-row diatomic hydrides

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Abstract
Diagrammatic many‐body perturbation theory is formulated through third order and applied to LiH, BH, and HF with various sizes of two‐center Slater orbital basis sets. The most extensive calculations use 46 orbitals to recover 94, 95, and 97% of the experimental correlation energy for the three molecules, respectively, when the perturbation expansion is carried through third order with pair restrictions and including selections of higher‐order diagrams via denominator shifts. A detailed analysis of the ’’pair’’ correlation energies relative to SCF occupied orbitals is given, including both inter‐ and intrapair contributions for the different spin cases.