Longitudinal wavevector- and frequency-dependent dielectric constant of the TIP4P water model

A computer adapted theory for self-consistent calculations of the wavevector- and frequency-dependent dielectric constant for interaction site models of polar systems is proposed. A longitudinal component of the dielectric constant is evaluated for the TIP4P water model in a very wide scale of wavenumbers and frequencies using molecular dynamics simulations. It is shown that values for the dielectric permittivity, calculated within the exact interaction site description, differ in a characteristic way from those obtained by the point dipole approximation which is usually used in computer experiment. It is also shown that the libration oscillations, existing in the shape of longitudinal time-dependent polarization fluctuations at small and intermediate wavevector values, vanish however for bigger wavenumbers. A comparison between the wavevector and frequency behaviour of the dielectric constant for the TIP4P water and the Stockmayer model is made. The static screening of external charges and damping of longitudinal electric excitations in water are considered as well. A special investigation is devoted to the time dependence of dielectric quantities in the free motion regime.