A hexagonal structure of solid molecular hydrogen with P6122 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 P6122. The P6122 and C2/c structures are very similar and their Raman and infra-red data are in good agreement with experiment. However, our calculations show that the hexagonal P6122 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 P6122 below about 200 GPa and monoclinic C2/c at higher pressures.
@article{arxiv.1609.07486,
title = {Hexagonal structure of phase III of solid hydrogen},
author = {Bartomeu Monserrat and Richard J. Needs and Eugene Gregoryanz and Chris J. Pickard},
journal= {arXiv preprint arXiv:1609.07486},
year = {2016}
}