Application of quasidegenerate many-body perturbation theory to the calculation of molecular excited valence state negative ion Feshbach resonances

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Abstract
Quasidegenerate many‐body perturbation theory (QDMBPT) is shown to generate a good method for representing the abstract Feshbach projectors, required in the evaluation of the energies of negative ion resonances, provided the valence space is sufficiently large to produce a good zeroth order description of these valence state resonances. Our recent advances in extending third order QDMBPT to large valence spaces are utilized to calculate potential curves and spectroscopic constants for the lowest four excited states of CH− and the vertical electron affinities to excited states of NH− and OH−. The experimentally assigned a 1Δ excited state of CH− is confirmed by the calculations, and a few more bound resonant molecular states are predicted. A simple one electron molecular orbital model is extracted from the large scale calculations to describe the zeroth order electronic structure of the excited states of CH−, NH−, and OH−.

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