Structures of hydrogen at megabar pressures

The atomic and molecular phases of hydrogen at megabar pressures are studied with use of the band theoretical calculation based on the local density approximation. The free energies, in the atomic phase of the hydrogen, of structures in a tetragonal diamond family are calculated and compared with those of structures studied earlier. The Cs-IV structure is of lowest free energy at pressures from 360 GPa to 1.7 TPa. And the stable region for the $\beta$-Sn structure is narrow, which lies in the region from 260 GPa to 360 GPa. At pressures lower than 260 GPa, the molecular phase becomes stable. Possibilities of the metallization in the molecular phase are briefly discussed.