DERIVATION OF CLASS-II FORCE-FIELDS .4. VAN-DER-WAALS PARAMETERS OF ALKALI-METAL CATIONS AND HALIDE ANIONS

A critical survey of previously reported van der Waals parameters for alkali metal cations and halide anions is presented. A new set of forc e field parameters is proposed, derived by fitting the experimental la ttice constants and lattice energies of 20 ionic alkali halide crystal s. These parameters are constrained to satisfy two relationships conne cting the ions with the isoelectronic noble gases-the relative van der Waals radii R and the coefficients of the London dispersion energies C-6-using the experimentally determined noble gas van der Waals param eters. In addition to reproducing physical trends in common with atoms of isoelectronic species, the present parameters predict more accurat e crystal structures and energies and, when combined with a molecular force field for water, also quite accurate gas-phase ion-water interac tion energies and aqueous solution structures compared to the computed results previously reported by other authors.