Figure 2: Phase diagram of hydrogen.

Regions of stability for the molecular solid (yellow), molecular liquid (purple), atomic solid (blue) and atomic liquid (pink) are indicated by the various colours. The dashed line separating the molecular and atomic liquid phases is taken from quantum Monte Carlo calculations³⁵. The solid line separating the molecular solid and molecular liquid phases is taken from ab initio MD simulations³⁶, whose negative slope has been confirmed by experiment⁶. The thick black line is the melting curve obtained in this study from the ab initio PIMD coexistence simulations. The solid lines separating phases I, II, III and IV are from (refs 12,13). The inset shows how the high-pressure melting curve (dashed lines) are established here. The black and red triangles (inset) correspond to the PIMD and MD results, respectively. The solid up triangles give the highest temperatures for solidification and the solid down triangles show the lowest temperatures for liquefaction. At 900 and 1,200 GPa, the so-called degeneracy temperature is ~40 K, below which the exchange of nuclei will be important. Accordingly, 50 K was the lowest temperature examined in our PIMD simulations. At this temperature each simulation yields a liquid state, and so the two open triangles at 900 and 1,200 GPa indicate upper bounds for the melting temperature.