Charge-transfer energy in closed-shell ion–atom interactions

The importance of charge-transfer energy in the interactions between closed-shell ions and atoms is investigated. Ab initio calculations on H+–He and Li+–He are used as a guide for the construction of approximate methods for the estimation of the charge-transfer energy for more complicated systems. For many alkali ion–rare gas systems the charge-transfer energy is comparable to the induction energy in the region of the potential minimum, although for doubly charged alkaline-earth ions in rare gases the induction energy always dominates. Surprisingly, an empirical combination of repulsion energy plus asymptotic induction energy plus asymptotic dispersion energy seems to give a fair representation of the total interaction, especially if the repulsion energy is parameterized, despite the omission of any explicit charge-transfer contribution. More refined interaction models should consider the charge-transfer energy contribution.