  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 GPa to 300 GPa. On heating or pressurizing we find first-order
phase transitions at the experimental conditions of the phase III - IV
boundary. Direct comparison of Raman vibrons with experiment provides 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.

Materials Science