Studies of the optical-like high frequency dispersion mode in liquid water

An "optical"-like collective mode (OM) above 500 cm^-1, or ω=100 ps^-1, is known on the basis of molecular dynamics (MD) simulation of the TIP4P model for liquid water at T=293 K. Recently we proposed a dielectric theory of the OM for the same water model by applying the reference memory function approximation (RMFA), a theory of the computationally convenient type. It enables us to extend the MD results for the dielectric permittivity ∈_ω into the region of finite wave vector k. Thus we calculate the longitudinal dielectric function ∈_L(k, ω) and the corresponding correlation functions for collective fluctuations in local polarization charge densities and currents required to characterize the OM. In the work reported here, we find that the calculated OM and its dispersion relation are relatively insensitive to whether the RMFA is implemented with collective or single particle reference dynamics, the principal difference being a more structured OM in the latter case. This finding provides the justification for applying the RMFA with reference dynamics based on the MD results of Frattini et al. for the single particle dipole reorientation time correlation functions of the same water model at various temperatures. We find that the OM has a definite temperature dependence, but the effect is weak even in the temperature range of real supercooled water.