Large local energy fluctuations in water. II. Cooperative motions and fluctuations

Large local energy fluctuations in liquid water and their physical origin are investigated by using classical molecular dynamics (MD) calculation and quenching techniques. Performing a trajectory calculation of 100 ps, it is found that large rotational motions of individual water molecules, which are always associated with potential energy destabilization of 10-20 kcal/mol, occur once in about 10 ps. The stabilization and destabilization of the individual water molecules are induced by cooperative motions. In order to analyze these cooperative motions in the liquid water, the water structures are quenched to their local minima (called the inherent structures). Comparing the inherent structures successively visited by the system, it is found that collective motions of about 10-40 molecules localized in space occur in unstable regions. The potential energy fluctuation of an individual molecule can reach up to 15 kcal/mol even in the inherent structures. The strong potential energy correlation among neighboring molecules indicates these cooperative motions cause the "flip-flop"-type energy exchanges; as a molecule is stabilized, another is to be unstabilized and vice versa. A flip-flop motion does not involve a (large) energy barrier but causes large energy fluctuations of the individual molecules. A large portion of potential energy fluctuations of the individual water molecules is accounted for as the superposition of fluctuations in the inherent structures and those in the normal modes build upon these structures.