Effects of intermolecular charge transfer in liquid water on Raman spectra.

The low-frequency vibrational spectrum of liquid water is composed of contributions from the intermolecular librational and translation modes. The existence of these two modes introduces difficulty into the simulation of experimentally obtained Raman spectra. We constructed a polarizability function for a water model that includes intramolecular charge flow (CF) effects, intermolecular charge transfer (CT) effects, and intermolecular dipole-induced-dipole (DID) effects. We computed the one-dimensional (ID) Raman and terahertz (THz) spectra with all of these effects included (CFCT-DID) and compared with experimental spectra. We find that the CFCT-DID function provides a better description of the experimental results, because the CT effects reduce the polarizability only for translational motion to which parallelly polarized (VV) and perpendicularly polarized (VH) Raman spectra are sensitive. In our calculations of two-dimensional (2D) Raman and THz-Raman spectra, we observe the enhancement of echo signals in both cases. The details of the CFCT-DID function, along with its source code, are provided in the Supporting Information.