Surface affinity of the hydronium ion: the effective fragment potential and umbrella sampling.

The surface affinity of the hydronium ion in water is investigated with umbrella sampling and classical molecular dynamics simulations, in which the system is described with the effective fragment potential (EFP). The solvated hydronium ion is also explored using second order perturbation theory for the hydronium ion and the empirical TIP5P potential for the waters. Umbrella sampling is used to analyze the surface affinity of the hydronium ion, varying the number of solvent water molecules from 32 to 256. Umbrella sampling with the EFP method predicts the hydronium ion to most probably lie about halfway between the center and edge of the water cluster, independent of the cluster size. Umbrella sampling using MP2 for the hydronium ion and TIP5P for the solvating waters predicts that the solvated proton most probably lies about 0.5-2.0 Å from the edge of the water cluster independent of the cluster size.