Alkali halide force fields: a search for an acceptable compromise

We used three sets of individual interaction potentials for alkali and halide ions. The first was the model where a charge on spring dipole (CAS) expressed the polarization of the ion fitted to the hydration free energy, energy and geometry of ion-water clusters, and energy and density of ambient crystals (P.T. Kiss and A. Baranyai, J. Chem..Phys.,141, 114501 (2014)). The halide ions have considerable polarization but a mere dipole is unable to improve the quality of estimations for all possible states and transitions. In symmetric environment like in alkali halide crystals the dipole moment of ions is close to zero and not very different in melt. Thus, we can also test the nonpolarizable part of this model as a second version. A third model was a modified nonpolarizable force field, also producing correct ion-water clusters, but devised to estimate the melting temperature of alkali halide crystals. Using the third model we calculated the melting temperature and the density of melts. However, the need for higher order polarization is indicated by the difficulty to produce accurate melting temperatures for each pair of the 20 combinations of ions. The role of polarization was studied in ambient aqueous solutions of NaCl and RbI. All three interaction models were investigated at three different concentrations in BK3 water. While in the ion – single BK3 water clusters the contribution of the induced dipole of the ion is at most 5% of the energy, in aqueous solutions of RbI the polarizable model produces very different structures than its nonpolarizable counterparts. We evaluate the results and discuss problems towards a polarizable potential of individual alkali and halide ions