An overlap model for exchange-induction: application to alkali halides

An existing method for estimating intermolecular exchange energies in weak Van der Waals interactions, based on the overlap between their ground-state charge densities, is extended to ionic interactions using the overlap between the polarised charge density of a negative ion and the ground-state charge density of surrounding positive ions. This enables the exchange-induction energy to be included in the model. One adjustable parameter for each ion pair is fitted to the dimer interaction energy. The prediction of in-crystal anion polarizabilities is excellent for the five alkali halides tested, and polarizabilities of anions in ionic dimers and trimers are reasonably well reproduced for lithium halides.