  We present a new equation of state (EOS) for dense hydrogen/helium mixtures
which covers a range of densities from $10^{-8}$ to $10^6$ g.cm$^{-3}$,
pressures from $10^{-9}$ to $10^{13}$ GPa and temperatures from $10^{2}$ to
$10^{8}$ K. The calculations combine the EOS of Saumon, Chabrier & vanHorn
(1995) in the low density, low temperature molecular/atomic domain, the EOS of
Chabrier & Potekhin (1998) in the high-density, high-temperature fully ionized
domain, the limits of which differ for H and He, and ab initio quantum
molecular dynamics (QMD) calculations in the intermediate density and
temperature regime, 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 stellar or planetary evolution calculations. The EOS results are compared
with existing experimental data, namely Hugoniot shock experiments for pure H
and He, and with first principle 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.
