AB-INITIO CALCULATIONS FOR SOLID MOLECULAR-HYDROGEN

Ab initio calculations are used to propose a unified model for the pre ssure-temperature phase diagram of solid molecular hydrogen. Path inte gral Monte Carlo calculations model the orientational order-disorder t ransition of spatially symmetric molecules. Possible ordered molecular structures near 150 GPa pressure are examined in the local density ap proximation. It is shown that the discontinuity in the molecular vibro n frequency versus pressure observed in this regime can be attributed to a combined band overlap and orientational transition. However, an o rientationally inhomogeneous insulating phase may preempt the anticipa ted bulk transition along the 150 GPa experimental phase line. Finally , calculations suggest that the hcp semimetallic phase undergoes a mar tensitic transition to orthorhombic symmetry in the same region.