Calculation of free energy changes in ion–water clusters using nonadditive potentials and the Monte Carlo method

Abstract

A thermodynamic perturbation method, using a Monte Carlo procedure, has been applied to the calculation of the free energy differences between ion–water clusters containing a given number of water molecules (n=1,...,6) and different monovalent ions, i.e., Li+, Na+, K+, F−, and Cl−. To our knowledge, this is the first application of the free energy perturbation approach with a nonadditive intermolecular potential. The results confirm the usefulness of the method and yield free energy differences of hydration which are in generally very good agreement with experiment. Our calculations have also revealed some structural changes which take place within the first hydration shell when one ion is replaced by another in the hexahydrate clusters. For Li+ one obtains a well defined 4+2 mean structure, whereas for a larger ion such as K+ the solvation structure is closer to a 5+1 type. For anions, this tendency is reversed, i.e., for F− one finds a well defined four coordinate structure for a cluster with four water molecules, but for the larger anion Cl− the cluser tends to be a 3+1 structure. In addition, favorable water–water interactions are apparent in anion–water clusters, but not in cation–water clusters.