Layering transitions and the structure of dense hydrogen

Recent theories of the structure of solid hydrogen at high pressure have tended to focus on the hexagonal close-packed (hcp) class of structures typical of low pressure, the contribution of the zero-point motion often being neglected. Here we examine the energy of solid hydrogen at high pressure in different structures, using density-functional theory augmented by the self-consistent harmonic approximation. Above a relative compression rho/rho(0) similar to 9 we report an orthorhombic structure, Cmca, that has lower energy (both static and zero-point) than the lowest-energy hcp structures, indicative of a possible layering or martensitic transition occurring in the vicinity of the recently observed high infrared activity.