# Structure and phase boundaries of compressed liquid hydrogen

## TL;DR

This study uses first principles molecular dynamics to map the phase boundaries of compressed liquid hydrogen, revealing a molecular phase with short-range order and implications for hydrogen's equation of state.

## Contribution

It provides new insights into the molecular-atomic transition and phase structure of liquid hydrogen at high pressures, suggesting revisions to existing models.

## Key findings

- Existence of a short-range ordered molecular phase above 100 GPa
- Sharp molecular-atomic crossover with pressure drop
- Implications for hydrogen equation of state data

## Abstract

We have mapped the molecular-atomic transition in liquid hydrogen using first principles molecular dynamics. We predict that a molecular phase with short-range orientational order exists at pressures above 100 GPa. The presence of this ordering and the structure emerging near the dissociation transition provide an explanation for the sharpness of the molecular-atomic crossover and the concurrent pressure drop at high pressures. Our findings have non-trivial implications for simulations of hydrogen; previous equation of state data for the molecular liquid may require revision. Arguments for the possibility of a $1^{st}$ order liquid-liquid transition are discussed.