Structure and bandgap closure in dense hydrogen

The possibility that steadily compressed hydrogen might undergo a transition from a proton-paired insulator to a monatomic metal was first suggested in 1935 (ref, 1). But experimental realization of metallic hydrogen in solid form has remained elusive, despite studies at pressures as high as 342 GPa (ref. 2). The pairing structure is known to be robust (from the persistence of its associated vibron mode(3)), leading to the suggestion of an alternative route to the metallic state, involving a band-overlap transition in which the pairing is preserved(4). Here we report density functional calculations within the local density approximation that predict a range of densities for hydrogen where a paired or molecular metallic state may be energetically preferred. The transition to this metallic state is naturally associated with the closing of an overall bandgap; but the pressures required to effect the transition are shown to change significantly when the gaps are corrected by approximate inclusion of many-electron effects. The implication is that a complete resolution of the structural and phase problem in dense hydrogen may require methods beyond the local density approximation.