Molecular dissociation in hot, dense hydrogen

We present a path-integral Monte Carlo study of dissociation in dense hydrogen (1.75 less than or equal to r(s) less than or equal to 2.2 with r(s) the Wigner sphere radius). As the temperature is lowered from 10(5) 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.