Collective Dynamical Properties of Liquid Water

A theoretical and computer simulation analysis suggests that two collective modes can propagate in liquid water for wave vectors $k>\mathrm{}0.64\mathrm{}$ ${\mathrm{\AA }}^{-1\mathrm{}}$. The low-frequency mode (ordinary sound) turns out to be associated with the oxygen density fluctuations, whereas the hydrogen density fluctuations are found to be governed by both modes, thus indicating that the high-frequency collective motion is related to fast librational oscillations. This allows a simply physical interpretation of the existing experimental and computer simulation data.