Insulator-to-Metal Transition in High-pressure Solid Hydrogen from Diffusion Monte Carlo Method

The mechanism of insulator-to-metal transition (IMT) in solid hydrogen is still not fully understood despite extensive research efforts. Experimental investigations are hindered by difficulties in achieving the required pressure. As such, ab initio calculations have become indispensable in studying said mechanism. In this study, the mechanism of IMT is elucidated by examining the phase diagram of solid hydrogen between 300 GPa and 800 GPa. We relax the candidate structures using the stochastic self-consistent harmonic approximation (SSCHA) working in conjunction with density functional theory (DFT) to incorporate the anharmonic effects. The diffusion quantum Monte Carlo (DMC) method is employed to correct the electronic energy from DFT. To uncover novel structures, a crystal structure search is conducted. We take the established candidate of phase III (C2/c-24), phase VI (Cmca-12), and atomic metallic phase (I4₁/amd) for evaluation. Other candidates of phase VI which have only been evaluated under DMC and harmonic zero point energy, namely the Pbam-8 and P2₁/c-8, are also included. Our calculations show that both Pbam-8 and P2₁/c-8 become less favorable than Cmca-12 upon the inclusion of anharmonicity. Moreover, our structure search did not find any competitive structures. These results support the current established sequence of phase transitions in solid hydrogen from its insulating to atomic metallic phase: C2/c-24 -> Cmca-12 -> I4₁/amd.