Candidate structure for the H$_2$-PRE phase of solid hydrogen

TL;DR

This study proposes a new candidate structure, P2_{1}/c-8, for the H$_2$-PRE phase of solid hydrogen at high pressures, using advanced computational methods to address longstanding structural debates.

Contribution

The paper introduces a novel candidate structure for the H$_2$-PRE phase based on particle swarm optimization and Monte Carlo simulations, advancing understanding of high-pressure solid hydrogen.

Findings

P2_{1}/c-8 is the most stable structure at 400-500 GPa.

The proposed structure reproduces IR spectral features of the H$_2$-PRE phase.

The study combines DFT and diffusion Monte Carlo methods for stability confirmation.

Abstract

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 been 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, Saturn...) strongly depends on the properties of inner high-pressured hydrogen. This work describes new possible structures appearing under high pressures of 400$\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 $P2_{1}/c$-8 as a promising candidate structure for the H$_2$-PRE phase. $P2_{1}/c$-8 is predicted the most stable at 400 and 500~GPa. $P2_{1}/c$-8 reproduces qualitatively the IR spectrum peaks observed in the H$_2$-PRE phase.