Ab initio calculations for solid molecular hydrogen

Ab initio calculations are used to propose a unified model for the pressure-temperature phase diagram of solid molecular hydrogen. Path integral Monte Carlo calculations model the orientational order-disorder transition of spatially symmetric molecules. Possible ordered molecular structures near 150 GPa pressure are examined in the local density approximation. It is shown that the discontinuity in the molecular vibron frequency versus pressure observed in this regime can be attributed to a combined band overlap and orientational transition. However, an orientationally inhomogeneous insulating phase may preempt the anticipated bulk transition along the 150 GPa experimental phase line. Finally, calculations suggest that the hcp semimetallic phase undergoes a martensitic transition to orthorhombic symmetry in the same region.