Ab initio EOS for water-hydrogen mixtures

In addition to computing the pressure and the internal energy, we derive the Gibbs free energy by performing a thermodynamic integration. For all conditions under consideration, our simulations predict hydrogen and water to mix in all proportions.

The thermodynamic behavior of the mixture can be well described with an ideal mixing approximation. We suggest that a substantial fraction of water and hydrogen in giant planets may occur in homogeneously mixed form rather than in separate layers. The extent of mixing depends on the planet's interior dynamics and its conditions of formation, in particular on how much hydrogen was present when icy planetesimals were delivered.