Molecular dissociation in hot, dense hydrogen

We present a path-integral Monte Carlo study of dissociation in dense hydrogen (1.75 ≤ r_s ≤ 2.2, with r_s the Wigner sphere radius). As the temperature is lowered from 10⁵ to 5000 K, a molecular hydrogen gas forms spontaneously from a neutral system of protons and electrons. At high density, r_s < 2.0, thermally activated dissociation is accompanied by decreasing pressure, signaling the presence of a first order transition and critical point. The decrease in electron kinetic energy during dissociation is responsible for the pressure decrease and transition. At lower density the phase transition disappears.