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Anomalous acoustic dynamics in liquid water: the origin of the low frequency mode

A high frequency sound mode has been observed in neutron scattering experiment and molecular dynamics (MD) simulations of liquid water. The associated sound velocity is about 3500 m s-1, which is more than twice the hydrodynamic sound velocity. Two explanations have been put forward to explain this phenomenon. One is that the high sound velocity is associated with a new excitation called ‘fast sound’ which is distinct from the ordinary sound mode. The other is that the high sound velocity is a result of a continuous positive dispersion of the ordinary sound mode as the wavevector increases from the hydrodynamic regime. Recent MD simulation of liquid water and a water cluster (H2O)108 has shown that two peaks exist in the longitudinal current spectrum of oxygen atoms. The high and low frequency peaks have been assigned as the fast and the ordinary sound modes, respectively. In this paper, we look for the origin of the low frequency peak in the spectrum. By separating the longitudinal current spectrum of ma...