Hexagonal structure of phase III of solid hydrogen

A hexagonal structure of solid molecular hydrogen with P6(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 P6(1)22. The P6(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 P6(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 P6(1)22 below about 200 GPa and monoclinic C2/c at higher pressures.