3-BODY FORCES EFFECT IN MONTE-CARLO STUDIES OF PROTONATED HYDRATES

The effect of three-body forces in Monte Carlo studies of protonated h ydrates is shown on several properties: clustering energies, coordinat ion number, possibility of exchange of water molecules between the she lls, distribution of the water molecules around the oxonium ion, geome try of energetic minima. The main feature is that some geometrical cof igurations are artificially favoured with additive potentials. This in volves qualitative and quantitative effects. The clustering energies f or small clusters are not correctly described if the non-additive cont ribution is not taken into account. Additive potentials favour an easy exchange between water molecules of the first two solvation shells, w hile such an exchange is only occasional if nonadditivity is considere d. The distributions of the water molecules around the ion are differe nt with additive and non-additive potentials. This explains, in partic ular, why additive potentials can lead to erroneous coordination numbe rs.