Vibrational Modes of Hydrogen Hydrates: A First-Principles Molecular Dynamics and Raman Spectra Study

We have employed classically propagated molecular dynamics (MD), within the framework of density functional theory (DFT), to calculate vibrational spectral band of molecular hydrogen trapped in clathrate hydrate, with large-cage occupancy from 1 to 4, at similar to 260 K and similar to 2 kbar. The predicted vibrations, obtained by applying a state-of-the-art generalized gradient approximation (GGA) functional with nonlocal correlation (VdW-DF), reproduce satisfactorily our own accurate Raman spectra (at the same temperature and pressure conditions). We decomposed the MD-sampled vibrational band to individual peaks and assigned them to the vibration of H-2 molecules enclosed in small and large cages of SII hydrate. By summing the resulting spectral bands, we have demonstrated that the measured spectral response is a complex composition of signals originating from H-2 molecules experiencing different local, intracage environments.