Nonlocal Dielectric Properties of Water: The Role of Electronic Delocalization

The nonlocal dielectric properties of liquid water are studied in the context of <i>ab initio</i> molecular dynamics (AIMD) simulations based on the density functional theory. We calculate the dielectric response from the charge structure factor of the liquid using the fluctuation-dissipation theorem. We show that the dielectric response function of <i>ab initio</i> simulations differs significantly from that of classical force-fields, both qualitatively and quantitatively. In particular, it exhibits a larger amplitude and a wider range of responding wave numbers. We suggest that the difference is due to the localization of the electronic charge density inherent in both classical force files and Wannier post-treatment of DFT densities. The localized charge models do not reproduce the shape of the response function even for <i>q</i> corresponding to intermolecular distances, and could lead to a significant underestimation of the dielectric response of the liquid by a factor of 10.