Candidate structure for the H2 -PRE phase of solid hydrogen

Experimental progress finally reached the metallic solid hydrogen phase, which was predicted by Wigner and Huntington over 80 years ago. However, the different structures in the phase diagram are still being debated due to the difficulty of diffraction experiments for high-pressured hydrogen. The determination of crystal structures under extreme condition is both of the basic condensed matter physics, and in planetary science: the behavior of giant gaseous planets (e.g., Jupiter and Saturn) strongly depends on the properties of inner high-pressured hydrogen. This work describes possible structures appearing under high pressures of $400\ensuremath{\sim}600$ GPa. We applied a structural search using particle swarm optimization with density functional theory (DFT) to propose several candidate structures. For these structures, we performed fixed-node diffusion Monte Carlo simulations combined with DFT zero-point energy corrections to confirm their relative stability. We found $P{2}₁/c\text{\ensuremath{-}}8$ as a promising candidate structure for the ${\mathrm{H}}₂$-PRE phase. $P{2}₁/c\text{\ensuremath{-}}8$ is predicted the most stable at 400 and 500 GPa. $P{2}₁/c\text{\ensuremath{-}}8$ reproduces qualitatively the IR spectrum peaks observed in the ${\mathrm{H}}₂$-PRE phase.