Stable phases of solid hydrogen at megabar pressures and at zero temperature

We have explored the stable phases of solid hydrogen by calculating band-theoretical energies in the local density approximation, for the system of oriented molecules at Mbar pressures and at zero temperature. It is shown that the hexagonal-close-packed lattice persists up to ∼300 GPa with a structure of space group Pca$2_1$, which has wide band gaps. The subsequent phase is still molecular with a rutile structure with c/a∼0.9 which must be conducting. The molecular phase terminates at ∼600 GPa, where the atomic phase with a planar structure appears.