Analysis of the bonding in alkali-cation/Rg complexes (Rg = He-Xe) using a simple model potential

We have applied the model-potential analysis of Breckenridge and co-workers to the Alk+/Rg systems, where Alk is an alkali atom and Rg is a rare-gas atom. Three spectroscopic constants, Re, De and ωe, are employed to obtain the potential, with fitted parameters being the A and b repulsive Born-Mayer potential parameters, and the effective charge on the metal center, Z. From the analysis, it is concluded that all of these systems can be described in physical terms, with the calculated charge on the metal center being very close to 1.00. Interestingly the effective charge on the metal atom from such model-potential calculations is consistently very slightly greater than 1.00. We discuss and investigate several possible sources for this small anomaly, including a minor breakdown of the model at small internuclear separations, where the “lengths” of the induced multipoles become significant compared to the internuclear separation R. Despite this small breakdown, overall the model is remarkably successful in reproducing high-level ab initio curves for the titular complexes over wide ranges of internuclear distances.