Degenerate many-body perturbation theory: Excited states of H2

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Abstract
The degenerate many‐body perturbation theory in Brandow’s diagrammatic expansion is applied to 16 excited states of H2 at two internuclear separations. A single set of molecular orbitals at each separation is used for all states. Energy corrections up to third order (inclusive) are calculated, and higher‐order effects are estimated by geometric extrapolation. Results agree with best available values to 1–2 kcal/mole. Differences between Goldstone and Brandow diagrams are discussed.