Collective motions in computer models of water. Large-scale and long-time correlations

Two-particle correlation functions describing the simultaneous motion of a pair of molecules initially separated by a given distance R0 are calculated to study collective effects in the diffusive motion of water molecules in molecular dynamics models. Various types of such functions and their dependences on the interaction potential, temperature, and the number of particles in the model are considered. At short times (of the order of ten picoseconds), these functions exhibit irregular behavior depending on R0. The most nontrivial and unexpected result was the detection of correlations in the displacements of pairs of particles that extend for tens of angstroms and last for hundreds of picoseconds. Such correlations are not observed in the random walk models of noninteracting particles. It is suggested that the observed large-scale correlations reveal the vortex-like motions of the molecules.