Stable solid molecular hydrogen above 900K from a machine-learned potential trained with diffusion Quantum Monte Carlo

TL;DR

This study uses a machine-learned potential trained with Quantum Monte Carlo data to explore high-pressure molecular hydrogen, discovering new stable phases and detailed phase transitions at high temperatures and pressures.

Contribution

It introduces a novel machine-learned interatomic potential trained with Quantum Monte Carlo, enabling accurate phase diagram exploration of hydrogen at extreme conditions.

Findings

Discovery of two new stable phases with Fmmm-4 structure.

Identification of a reentrant melting line with a maximum at 1450K and 150GPa.

Observation of a liquid-liquid transition around 1200K and 200GPa.

Abstract

We survey the phase diagram of high-pressure molecular hydrogen with path integral molecular dynamics using a machine-learned interatomic potential trained with Quantum Monte Carlo forces and energies. Besides the HCP and C2/c-24 phases, we find two new stable phases both with molecular centers in the Fmmm-4 structure, separated by a molecular orientation transition with temperature. The high temperature isotropic Fmmm-4 phase has a reentrant melting line with a maximum at higher temperature (1450K at 150GPa) than previously estimated and crosses the liquid-liquid transition line around 1200K and 200GPa.