  Liquid-liquid phase transition of hydrogen is at the center of hydrogen phase
diagram as a promising route towards emergent properties such as the
Wigner-Huntington metallization, superconductivity, and superfluidity. Here we
report a study on the liquid-liquid phase transition of hydrogen using the
state-of-the-art diffusion quantum Monte Carlo and density functional theory
calculations. Our results suggest that the metallization process happens at
lower pressures and temperatures compared to the structural phase transition of
molecular to atomic hydrogen. The consequence is that metallized molecular
hydrogen is stable at a wide range of pressures and temperatures. Our study
breaks the conventional assumption that metallization coinciding with
dissociation of hydrogen molecule, and the molecular metallic hydrogen liquid
phase is likely to become the frontier of studying hydrogen phase transitions.
