Resolving the Nature of the Lowest-Frequency Raman Mode of Liquid Water

The lowest-frequency Raman mode of water, observed through depolarized light scattering or optical Kerr effect techniques, is routinely used to track dynamic changes of interfacial water near ions or biomolecules. Despite this broad importance, its microscopic origin and relation to dielectric relaxation remains debated since decades─with conflicting interpretations even for pure water. To resolve this controversy, we first accurately compute the Raman spectrum down to a few GHz based on ab initio simulations. Second, we introduce detailed decomposition analyses down to the dynamics of nuclei and electrons to reveal that the rotational and translational contributions are equally important. Third, strong negative orientational cross-correlations as well as internal field effects are found to modify the rotational component in very distinct ways. Overall, our analysis down to the electronic structure level provides an unexpectedly complex mechanistic scenario that fully resolves the long-standing debate concerning the lowest-frequency Raman mode of water and its relation to the dielectric spectrum.