CRYSTAL-STRUCTURE OF MOLECULAR-HYDROGEN AT HIGH-PRESSURE

We have studied oriented phases of molecular hydrogen on the hcp lattice using quantum Monte Carlo methods, treating both electrons and protons as quantum particles. We find that for a wide range of pressures above approximately 123 GPa (rs=1.57), structures with molecules at an angle to the c axis are favored over the structure with molecules aligned along the c axis. For pressures greater than 150 GPa, where phase III is observed, the most stable structures which we have found are insulating, with molecules having an average orientation angle of 60°with respect to the c axis. This conclusion is consistent with recent observations of strong ir activity, which is forbidden in the aligned structure. Consequences for the phase diagram are discussed. © 1995 The American Physical Society.