  The atmospheres of cool, helium-rich white dwarfs constitute an exotic and
poorly explored physical regime of stellar atmospheres. Under physical
conditions where the temperature varies from $\rm 1000K$ to $\rm 10000K$, the
density can reach values as large as $\rm 2 g/cm^{3}$, and the pressure is as
high as $\rm 1 Mbar$, the atmosphere is no longer an ideal gas and must be
treated as a dense fluid. Helium atoms become strongly correlated and
refraction effects are present. Opacity sources such as $\rm He^{-}$ free-free
absorption must be calculated with a formalism that has never been applied to
astrophysical opacities. These effects have been ignored in previous models of
cool white dwarf atmospheres.
