The electronic structure of weakly bound systems. I. Rare‐gas bimolecular cations

A general perspective is presented of bimolecular positive ions formed from two rare gases that share a positive charge. These species, which are important in ionospheric processes, arise when neutral van der Waals species are ionized. A general theoretical program based on standard theoretical techniques (e.g., various configuration interaction and Møller‐Plesset techniques and quadratic configuration interaction) is outlined, and results for the geometry, stability, and vibrational frequencies for a set of rare gas dimers, AB+, are presented. Specifically, the molecules HeHe+, HeNe+, HeAr+, NeNe+, NeAr+, and ArAr+ are considered; the equilibrium geometry of the ground electronic state is determined at several levels of approximation. Then binding energies are calculated in several ways, and these binding energies are compared to experimental and other sophisticated theoretical results. In general, the agreement with experiment is good, within a few kcal/mol. Harmonic vibrational frequencies at the MP2 level are also computed.