  Due to their high photospheric density, cool helium-rich white dwarfs
(particularly DZ, DQpec and ultracool) are often poorly described by current
atmosphere models. As part of our ongoing efforts to design atmosphere models
suitable for all cool white dwarfs, we investigate how the ionization ratio of
heavy elements and the H$_2$-He collision-induced absorption (CIA) spectrum are
altered under fluid-like densities. For the conditions encountered at the
photosphere of cool helium-rich white dwarfs, our ab initio calculations show
that the ionization of most metals is inhibited and that the H$_2$-He CIA
spectrum is significantly distorted for densities higher than 0.1 g/cm$^3$.
