Short‐Wavelength Collective Dynamics in Metastable Water A Molecular Dynamics Simulation Study

Motivated by a controversial discussion of the short‐wavelength collective dynamics in water, we calculate the coherent dynamic structure factor from the oxygen motion in low‐density water. In accord with early results of Rahman and Stillinger, we detect at the lowest accessible wavevectors the simultaneous existence of two Brillouin lines. Whereas the low‐frequency mode is strongly damped and is considered to be a remnant of the ordinary sound, the high‐frequency mode can be attributed to collective excitations, propagating within the hydrogen‐bond network. Consequently, a density decrease lowers the weak damping of this mode even more, due to the increasing rigidity of the hydrogen‐bond network. For comparison, we also discuss the temperature and density dependence of a related single particle property, the frequency spectrum of the velocity autocorrelation function.