Equation of state calculations of hydrogen-helium mixtures in solar and extrasolar giant planets

Using density-functional molecular dynamics simulations, we study the equation of state for hydrogen-helium mixtures at conditions of giant planet interiors of 0.2-2.3 g cm(-3) and 1000-80 000 K for a typical helium mass fraction of 0.245. In addition to computing internal energy and pressure, we determine the entropy by applying a thermodynamic integration technique in the presence of electronic excitations. We extend this technique to include the dissociations and recombination of hydrogen molecules. Results are compared to the widely used analytic model by Saumon and Chabrier. We also report evidence of spontaneous hydrogen-helium phase separation in simulations at low temperature and high density. DOI: 10.1103/PhysRevB.87.014202