  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.

Chemical Physics;Computational Physics