Atomic and Molecular Phases of Hydrogen at Megabar Pressures

The stable structures of the solid hydrogen have been studied by the band theoretical calculation using the plane wave basis in the local density approximation. Comparing the Gibbs free-energies for candidate structures in the atomic and molecular phases, we have discussed the stable strcutures. The metallization occurs around 300 GPa by the molecular dissociation. We have also studied the pressure dependences of the vibrational frequencies at the Г-point in the molecular phase by the frozen phonon method in the harmonic approximation. Our results suggest that the molecules sit on the hcp sites and are canted from the c-axis.