Molecular dynamics study of the effects of ions on water microclusters

We report a molecular dynamics study of water ion microclusters. This study concerns clusters containing Na+, Rb+, Br−, and I− and up to eight water molecules, and larger clusters containing Cs+ and F− and up to 29 water molecules. For each cluster we calculated the total cluster energy, radial density functions, dipole orientations of water molecules around the ion, and effective ionic radii. The ion lowers the cluster energy by a considerable amount, depending to a great extent on the ionic radius. The ion strongly orients all of the water molecules in the cluster. The coordination numbers of the ions depend primarily on ion size and to a lesser extent on charge sign. Our calculated effective ionic radii are in reasonable agreement with the Pauling radii.