Frequency and wave-vector dependent dielectric function of water: Collective modes and relaxation spectral

The longitudinal frequency and wave-vector dependent complex dielectric response function chi(k, omega) = 1 - 1/epsilon(k, omega) is calculated in a broad range of k values-by means of molecular dynamics computer simulation for a central force model of water. Its imaginary part, i.e., Im{E(k, omega)}//epsilon(k, omega)/(2), shows two main contributions in the region of small k values: Debye-like orientational relaxation in the-lower;frequency part of the spectrum and a damped librational resonance at the high frequency wing. The Debye relaxation time does not follow a de Gennes-like pattern: tau(k) goes through a maximum at k approximate to k* approximate to 1.7 Angstrom(-1), while the static polar structure factor S(k) peaks at k approximate to 3 Angstrom(-1). The resonance frequency omega(k) and the decay decrement gamma(k) show a dispersion law, indicative of a decaying optical-like mode; the libron. With an approximate normal mode approach, we analyze the origin of this mode on a molecular level which shows that it is due to a damped propagation of molecular orientational. vibrations through the network of hydrogen; bonds. At high k the decay, due to dissipation of collective into single particle motions, dominates. The static dielectric function is calculated on the basis of the response function spectra via the Kramers-Kronig relation. In the small k region epsilon(k) decreases from the macroscopic value E approximate to 80 to a value approximate to 15, i.e. it exhibits a Lorentzian-type behavior. This behavior is shown to be determined by higher order multipole correlation functions. In the intermediate and high k range, our results on epsilon(k) and chi(k) are in excellent agreement with data extracted from experimental partial pair correlation functions: epsilon(k) exhibits two divergence points on the k axis with a range of negative, values in between where a maximum in chi(k) is found: with chi(max)(k) much greater than 1, indicative-of overscreening. Consequences of quantum corrections to chi(k) with respect to a purely classical calculation are discussed and consequences are shown for the interaction energy between hydrated ions. (C) 1998 American Institute of Physics.