  The properties of hot dense helium at megabar pressures were studied with two
first-principles computer simulation techniques, path integral Monte Carlo and
density functional molecular dynamics. The simulations predicted that the
compressibility of helium is substantially increased by electronic excitations
that are present in the hot fluid at thermodynamic equilibrium. A maximum
compression ratio of 5.24(4)-fold the initial density was predicted for 360 GPa
and 150000 K. This result distinguishes helium from deuterium, for which
simulations predicted a maximum compression ratio of 4.3(1). Hugoniot curves
for statically precompressed samples are also discussed.
