Study of sound propagation in liquid water, which has been stimulated by the experimental observation of a fast propagating mode in the high wavevector region, has become a test-case in the developing field of collective excitations in molecular liquids. Here we present results obtained from molecular dynamics simulations of liquid water in a large wavevector range (from 0.026 to 1. °A −1) covering the range where the sound velocity changes from values comparable to hydrodynamic sound velocity to the values observed by neutron scattering. On the basis of such data, we discuss the extension of visco-elastic theory developed for simple liquids to molecular liquids. We also discuss the possibility of a novel phenomenon, related to the crossing of acoustic and optical branches in the wavevector range in which the behavior of the liquid has a significant elastic component.
