Raman Spectra of Liquid Water from <i>Ab Initio</i> Molecular Dynamics: Vibrational Signatures of Charge Fluctuations in the Hydrogen Bond Network

We report the first <i>ab initio</i> simulations of the Raman spectra of liquid water, obtained by combining first principles molecular dynamics and density functional perturbation theory. Our computed spectra are in good agreement with experiments, especially in the low frequency region. We also describe a systematic strategy to analyze the Raman intensities, which is of general applicability to molecular solids and liquids, and it is based on maximally localized Wannier functions and effective molecular polarizabilities. Our analysis revealed the presence of intermolecular charge fluctuations accompanying the hydrogen bond (HB) stretching modes at 270 cm^–1, in spite of the absence of any Raman activity in the isotropic spectrum. We also found that charge fluctuations partly contribute to the 200 cm^–1 peak in the anisotropic spectrum, thus providing insight into the controversial origin of such peak. Our results highlighted the importance of taking into account electronic effects in interpreting the Raman spectra of liquid water and the key role of charge fluctuations within the HB network; they also pointed at the inaccuracies of models using constant molecular polarizabilities to describe the Raman response of liquid water