Relaxing bulk modulus in water: Evidence from computer simulation of instantaneous compression.

Water demonstrates numerous anomalies. One of them is the extremely large strength of positive sound dispersion (PSD), which means that the frequency dependent speed of sound strongly exceeds the hydrodynamics one. While in simple atomic melts the strength of PSD is of the order of dozen percents, in molecular liquids it is up to $50 \%$, it reaches as much as $200 \%$ in water. The origin of such enormous PSD in water is discussed in the present paper. In the framework of molecular simulation methods it is shown that large PSD of water and supercooled silicon (two examples of anomalous liquids) is related to presence of slowly relaxing processes in such liquids which lead to strong frequency dependence of the bulk modulus.