Path integral Monte Carlo calculations of orientational ordering in compressed H2

Pressure-induced orientational ordering transitions in solid molecular hydrogen and its isotopes are determined by path integral Monte Carlo methods. Effective interactions among molecules in the solid are calculated in the local density approximation. Unlike previous calculations which have systematically underestimated the ordering densities for this system, the present-treatment affords a calculation of the phase diagrams of H2 and D2 in excellent agreement with experiment for orientational ordering up to a pressure of 100 GPa