Hexagonal structure of phase III of solid hydrogen
A hexagonal structure of solid molecular hydrogen with $P{6}_{1}22$ symmetry is calculated to be more stable below about 200 GPa than the monoclinic $C2/c$ structure identified previously as the best candidate for phase III. We find that the effects of nuclear quantum and thermal vibrations play a central role in the stabilization of $P{6}_{1}22$ The $P{6}_{1}22$ and $C2/c$ structures are very similar and their Raman and infrared data are in good agreement with experiment. However, our calculations show that the hexagonal $P{6}_{1}22$ structure provides better agreement with the available x ray diffraction data than the $C2/c$ structure at pressures below about 200 GPa. We suggest that two phase III like structures may be formed at high pressures: hexagonal $P{6}_{1}22$ below about 200 GPa and monoclinic $C2/c$ at higher pressures.