THE FREQUENCY AND WAVELENGTH DEPENDENT DIELECTRIC PERMITTIVITY OF WATER

The static and dynamic dielectric behaviour of a model of liquid water is studied in the supercooled region, as a function of wavelength. The data used for this analysis were obtained in a previous molecular dynamics simulation of a sample of 343 water molecules, modelled by the TIP4P potential, at 245 K. A substantial wavelength dependence of the static dielectric permittivity is observed, while the value found for epsilon(0) seems to indicate that the TIP4P model is able to account qualitatively for the increase of dielectric constant upon temperature decrease, as found in real water. The time correlation function of the longitudinal and transverse components of the dipole density as well as that of the individual and total dipole moment is also calculated, to relate collective dielectric properties to the single molecule relaxation. The collective and single molecule dielectric relaxation times are obtained and their ratio seems to be close to a static property, namely the short-range orientational correlation factor, g(s)(k). The frequency dependence of the dielectric constant is compared with previous simulation and experimental values. Furthermore, the longitudinal and transverse components of the hydrogen current are discussed. The molecular symmetry relates the latter properties to the corresponding component of the dipole density in the frequency range 80-200 THz, typical of the librational motions of water. From these results, a simple and unifying picture of the dynamics underlying the three dielectric bands of water in the frequency range 1-200 THz emerges.