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 ($r_s=1.57\mathrm{}$), 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.