A dielectric theory of the optical-like high-frequency mode in liquid water

The "optical"-like collective mode (OM) in TIP4P model water recently reported by Ricci et al. is investigated here with a simple dynamical theory, the reference memory function approximation (RMFA), applied to calculate the dynamical dielectric properties of rigid and nonpolarizable interaction site models for water. Our results indicate that the OM is responsible for the high-frequency local charge density fluctuations that are germane to the frequency- and wave vector-dependent longitudinal dielectric function of water. The results for the OM in the TIP4P and SPC/E water models studied here under RMFA are in good agreement with each other as well as with molecular dynamics simulation results of Ricci et al. on the TIP4P model. An interesting connection to ion solvation dynamics is established.