High-pressure structures of solid hydrogen: Insights from ab initio molecular dynamics simulations

Understanding the structural behavior of solid hydrogen under high pressures is crucial for uncovering its unique properties and potential applications. In this study, starting from the phase I of solid hydrogen-free-rotator hcp structure, we conduct extensive ab initio molecular dynamics calculations to simulate the cooling, heating, and equilibrium processes within a pressure range of 80-260 GPa. Without relying on any structure previously predicted, we identify the high-pressure phase structures of solid hydrogen as P2(1)/c for phase II, P6(5)22 for phase III, and BG(1)BG(2)BG(3) six-layer structure for phase IV, which are different from those proposed previously using the structure-search method. The reasonability of these structures are validated by Raman spectra and x-ray diffraction patterns by comparison with the experimental results. Our results actually show pronounced changes in the c/a ratio between phases I, III, and IV, which hold no brief for the experimental interpretation of an isostructural hcp transformations for phases I-III-IV.