Effect of an ionic impurity on the caloric curves of water clusters

The equilibrium heat capacities of model pure and heterogeneous water clusters have been calculated using exchange Monte Carlo simulations. For the pure water Cluster (H(2)O)(20), microcanonical and canonical caloric curves obtained from various rigid intermolecular potentials indicate the onset of melting to lie in the range 140-180 K, in reasonable agreement with previous estimates. Clusters doped with a single hydronium or ammonium impurity show a, significant shift of the melting point in the 20-molecule system, but a reduced effect when 50 molecules are reached.