Electronic states and charge-transfer mechanisms in solid molecular hydrogen

Abstract

Metallic behavior and the transition from a molecular to an atomic phase in solid hydrogen under pressure have been predicted theoretically and sought after experimentally for a long time. We report extensive first-principles investigations of the electronic states in static, hexagonal close packed (hcp) structures which are representative of solid molecular hydrogen at high pressure. In a structure which preserves the full hcp symmetry, charge is transferred from bonding to antibonding states at pressures near 1.5 Mbar. However, at still higher pressure charge transfer takes place in the opposite direction. In a different structure of lower symmetry and lower energy, antibonding states do not become occupied until much higher pressure ( ∼ 2.5 Mbar). The character of antibonding electronic states and their occupation with pressure provide insight into possible metallic behavior and the transition to the atomic phase.