Structural behavior and equation of state of atomic and molecular crystal phases of dense hydrogen at pressures up to 3.5 TPa are systematically investigated with density functional theory.The results indicate that the Vinet EOS model that fitted to low-pressure experimental data overestimates the compressibility of dense hydrogen drastically when beyond 500 GPa. Metastable multi-atomic molecular phases with weak covalent bonds are observed. When compressed beyond about 2.8 TPa, these exotic low-coordinated phases become competitive with the groundstate and other high-symmetry atomic phases. Using nudged elastic band method, the transition path and the associated energy barrier between these high-pressure phases are evaluated. In particular for the case of dissociation of diatomic molecular phase into the atomic metallic Cs-IV phase, the existent barrier might raise the transition pressure about 200 GPa at low temperatures. Plenty of flat and broad basins on the energy surface of dense hydrogen have been discovered, which should take a major responsibility for the highly anharmonic zero point vibrations of the lattice, as well as the quantum structure fluctuations in some extreme cases. At zero pressure, our analysis demonstrates that all of these atomic phases of dense hydrogen known so far are unquenchable.

No abstract