Diffusion Monte Carlo study of insulator-to-metal transition of high-pressure solid hydrogen

The phase diagram of solid hydrogen under high pressure remains a topic of high research interest. It is known that hydrogen becomes metallic at high pressure, however, the transition path it takes from insulator to metallic phase is still an open question. Investigating such a high-pressure regime experimentally is challenging due to the limitation in the currently achievable pressure and the difficulties in determining the crystal structure from characterization techniques. Consequently, computational approaches using accurate ab initio simulations have become indispensable tools to reliably model the phase diagram.

In this study, we focus on the phase diagram of solid hydrogen within the pressure range of 300 GPa to 800 GPa to investigate the mechanism of the insulator-to-metal transition. In addition to considering the previously discovered structures for evaluation, we employ a crystal structure search based on the evolutionary algorithm coupled with density functional theory (DFT) method to identify new potential candidate structures. A combination of DFT and stochastic self-consistent harmonic approximation (SSCHA) methods are utilized in the structural relaxation to account for the quantum anharmonic effects, and finally, the quantum diffusion Monte Carlo (DMC) calculation is performed to obtain the accurate static enthalpy.