FREQUENCY AND WAVE-VECTOR DEPENDENT DIELECTRIC FUNCTION OF WATER - COLLECTIVE MODES AND RELAXATION SPECTRAL

The longitudinal frequency and wave-vector dependent complex dielectri c response function chi(k, omega) = 1 - 1/epsilon(k, omega) is calcula ted in a broad range of k values-by means of molecular dynamics comput er simulation for a central force model of water. Its imaginary part, i.e., Im{E(k, omega)}//epsilon(k, omega)/(2), shows two main contribut ions in the region of small k values: Debye-like orientational relaxat ion in the-lower;frequency part of the spectrum and a damped libration al resonance at the high frequency wing. The Debye relaxation time doe s not follow a de Gennes-like pattern: tau(k) goes through a maximum a t k approximate to k approximate to 1.7 Angstrom(-1), while the stati c polar structure factor S(k) peaks at k approximate to 3 Angstrom(-1) . The resonance frequency omega(k) and the decay decrement gamma(k) sh ow a dispersion law, indicative of a decaying optical-like mode; the l ibron. 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 dam ped propagation of molecular orientational. vibrations through the net work of hydrogen; bonds. At high k the decay, due to dissipation of co llective into single particle motions, dominates. The static dielectri c function is calculated on the basis of the response function spectra via the Kramers-Kronig relation. In the small k region epsilon(k) dec reases from the macroscopic value E approximate to 80 to a value appro ximate to 15, i.e. it exhibits a Lorentzian-type behavior. This behavi or is shown to be determined by higher order multipole correlation fun ctions. In the intermediate and high k range, our results on epsilon(k ) and chi(k) are in excellent agreement with data extracted from exper imental partial pair correlation functions: epsilon(k) exhibits two di vergence points on the k axis with a range of negative, values in betw een where a maximum in chi(k) is found: with chi(max)(k) much greater than 1, indicative-of overscreening. Consequences of quantum correctio ns to chi(k) with respect to a purely classical calculation are discus sed and consequences are shown for the interaction energy between hydr ated ions. (C) 1998 American Institute of Physics.