  The photospheres of the coolest helium-atmosphere white dwarfs are
characterized by fluid-like densities. Under those conditions, standard
approximations used in model atmosphere codes are no longer appropriate.
Unfortunately, the majority of cool He-rich white dwarfs show no spectral
features, giving us no opportunities to put more elaborate models to the test.
In the few cases where spectral features are observed (such as in cool DQ or DZ
stars), current models completely fail to reproduce the spectroscopic data,
signaling shortcomings in our theoretical framework. In order to fully trust
parameters derived solely from the energy distribution, it is thus important to
at least succeed in reproducing the spectra of the few coolest stars exhibiting
spectral features, especially since such stars possess even less extreme
physical conditions due to the presence of heavy elements. In this paper, we
revise every building block of our model atmosphere code in order to eliminate
low-density approximations. Our updated white dwarf atmosphere code
incorporates state-of-the-art constitutive physics suitable for the conditions
found in cool helium-rich stars (DC and DZ white dwarfs). This includes new
high-density metal line profiles, nonideal continuum opacities, an accurate
equation of state and a detailed description of the ionization equilibrium. In
particular, we present new ab initio calculations to assess the ionization
equilibrium of heavy elements (C, Ca, Fe, Mg and Na) in a dense helium medium
and show how our improved models allow us to achieve better spectral fits for
two cool DZ stars, Ross 640 and LP 658-2.
