Structure of protonated water clusters: low-energy structures and finite temperature behavior

The structure of protonated water clusters H+(H2O)n (n=5-22) are examined by two Monte Carlo methods in conjunction with the OSS2 potential [L. Ojamae, I. Shavitt, and S. J. Singer J. Chem. Phys. 109, 5547 (1998)]. The basin-hopping method is employed to explore the OSS2 potential energy surface and to locate low-energy structures. The topology of the "global minimum," the most stable low-energy structure, changes from single ring to multiple ring to polyhedral cage as the cluster size grows. The temperature dependence of the cluster geometry is examined by carrying out parallel tempering Monte Carlo simulations. Over the temperature range we studied (25-330 K), all water clusters undergo significant structural changes. The trends are treelike structures dominating at high temperature and single-ring structures appearing in slightly lower temperatures. For n> or =7, an additional transition from single ring to multiple rings appears as the temperature decreases. Only for n> or =16 do polyhedral structures dominate the lowest temperature range. Our results indicate very dynamic structural changes at temperature range relevant to atmospheric chemistry and current experiments. The structures and properties of medium-sized protonated clusters in this temperature range are far from their global minimum cousins. The relevance of these findings to recent experiments and theoretical simulations is also discussed.