On the development of an effective model potential to describe water interaction in neutral and ionic clusters

Various effective components of the intermolecular interaction of water containing aggregates are examined and their modeling, in terms of the fundamental physical properties of the involved partners, is discussed. We focus, in particular, on the evolution of these components in going from the simplest neutral rare gas-water aggregates to bulk water and ionic water solutions. The analysis singled out that the model chosen to represent the van der Waals interaction as the composition of the action of three dispersion/induction-attraction centres and found to be appropriate to describe the lighter He-H2O and Ne-H2O systems, is not adequate to describe the heavier Ar-H2O aggregate. It was found, instead, that by increasing the mass of the rare gas, other short range contributions to the interaction come into play. Moreover, it was also found that the water molecule tends to behave as a single centre as the strength of the interaction increases. This led to the development of an effective model potential suitable to describe water clusters in the range going from gaseous to condensed phase. The role of electrostatic contributions is also evaluated. The proposed potential model is tested by comparing molecular beam scattering and neutron diffraction experiments with results of molecular dynamics (MD) calculations.