We present a new equation of state (EOS) for dense hydrogen/helium mixtures that covers a range of densities from 10?8 to , pressures from 10?9 to 1013 GPa, and temperatures from 102 to 108 K. The calculations combine the EOS of Saumon, Chabrier & van Horn in the low-density, low-temperature molecular/atomic domain, the EOS of Chabrier & Potekhin in the high-density, high-temperature fully ionized domain, the limits of which differ for H and He, and ab initio quantum molecular dynamics calculations in the regime of intermediate density and temperature, characteristic of pressure dissociation and ionization. The EOS for the H/He mixture is based on the so-called additive volume law and thus does not take into account the interactions between the two species. A major improvement of the present calculations over existing ones is that we calculate the entropy over the entire density–temperature domain, a necessary quantity for calculations of stellar or planetary evolution. The EOS results are compared with existing experimental data, namely Hugoniot shock experiments for pure H and He, and with first-principles numerical simulations for both the single elements and the mixture. This new EOS covers a wide range of physical and astrophysical conditions, from Jovian planets to solar-type stars, and recovers the existing relativistic EOS at very high densities, in the domains of white dwarfs and neutron stars. All the tables are made publicly available.
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