Understanding dielectric loss in water via distance-dependent dipole correlation functions

We have performed a molecular dynamics study which reveals that dielectric loss in liquid water in the gigahertz frequency regime does not predominantly arise from isolated molecular rotations, but rather from collective dipolar correlations spanning more than several tens of molecules. We quantified the spatial extent and temporal evolution of orientational fluctuations contributing to dielectric relaxation, by introducing a distance-dependent dipole correlation function. Three spatially distinct peaks were identified in the dipole vector correlation at 0.25 nm, 0.53 nm, and 0.75 nm. Combining these peaks with the coordination numbers obtained from the oxygen-oxygen radial distribution function reveals the coordinated reorientation of several tens of water molecules. These results demonstrate a strong link between the molecular structure and dielectric behavior, and show that the dominant dielectric loss in liquid water originates from coordinated dipolar dynamics extending over several coordination shells. This establishes a spatially resolved, microscopic connection between the dynamics of the hydrogen-bond network and the macroscopic dielectric response. While the characteristic frequencies estimated from the relaxation behavior of dipole correlation are not intended to be distinct, experimentally resolvable loss peaks, they provide shell-resolved, timescale indicators. The superposition consistently corresponds to the single, broad Debye relaxation observed experimentally in the gigahertz region.