Potential energy surfaces for water dynamics: Reaction coordinates, transition states, and normal mode analyses

Dynamics of water binding structure reorganization is investigated by analyzing the potential energy surfaces involved. The water structures in a trajectory are quenched to their local minima, called the inherent structures. The reaction coordinates, which connect the inherent structures successively visited by the system, are determined. It is found that the energy barrier heights, the transition state energies, along the reaction coordinates are mostly distributed in the range of 0.2-6 kcal/mol. The classification of inherent structures is made to groups of "overall-inherent structures"; successive inherent structures are most often not so geometrically distinct. It is found that transitions between the overall-inherent structures, involving large collective motions, occur in the subpicosecond time scale. Individual molecular motions in these collective motions are stongly correlated, not yielding large transition energies. The transition state energy sometimes reaches up to 20 kcal/mol, when the system goes through the ridge between deep minima, yielding ballistic dynamical behavior. Temperature dependence of the collective motions is also investigated.