Identification of high-pressure phases III and IV in hydrogen: Simulating Raman spectra using molecular dynamics

We present a technique for extracting Raman intensities from ab initio molecular dynamics (MD) simulations at high temperature. The method is applied to the highly anharmonic case of dense hydrogen up to 500 K for pressures ranging from 180 to 300 GPa. On heating or pressurizing we find first-order phase transitions under the experimental conditions of the phase III-IV boundary. At even higher pressures, close to 350 GPa, we find a second phase transformation to the previously proposed Cmca-4. Our method enables, for the first time, a direct comparison of Raman vibrons between theory and experiment at finite temperature. This turns out to provide excellent discrimination between subtly different structures found in MD. We find candidate structures whose Raman spectra are in good agreement with experiment. The new phase obtained in high-temperature simulations adopts a dynamic, simple hexagonal structure with three layer types: freely rotating hydrogen molecules, static hexagonal trimers, and rotating hexagonal trimers. We show that previously calculated structures for phase IV are inconsistent with experiment, and their appearance in simulation is due to finite-size effects.