Collective dynamics in three body water and sound dispersion

The collective properties of two and two plus three body models of liquid water are studied by molecular dynamics simulation. The three body forces retard the time decay of liquid structure and the diffusion of thermal energy, and also increase the viscosities. Positive dispersion in the sound velocity that is observed in the two body liquid for 0.25≤k≤0.6 Å−1 is even stronger in the three body liquid. From a comparison with various simple liquids it appears that the strength of the longer ranged attractive forces in the intermolecular potential is responsible for this phenomenon. The interpretation of recent experimental neutron scattering data is discussed in the light of the behavior of the simulated liquids and of a generalized hydrodynamics model, and no evidence is found for the existence of a second independent sound mode in water.