Limited effects of polarization for Cl−(H2O)<i>n</i> and Na+(H2O)<i>n</i> clusters

Monte Carlo statistical mechanics simulations have been carried out for Cl− and Na+ in clusters of 1–14 water molecules. OPLS potential functions were used including the TIP4P model of water that do not include explicit polarization effects. In contrast to a prior report with the same potential functions, the computed enthalpies of binding for Cl− are shown to agree closely with experimental data, while the values for Na+ are about 10% too exothermic. The latter discrepancy is largely removed by using Aqvist’s Na+ parameters with TIP3P water. Moreover, the structures of the clusters show the expected nonspherical distribution of water molecules.