  We present PHOENIX atmosphere models for metal-rich cool white dwarfs using
improved line shapes for the Na I and K I resonance doublets. Profiles for
collisional broadening due to H2 and He based on the adiabatic representation
show strong deviations from Van der Waals interaction at short distances.
Comparison with observed spectra that show extremely broadened Na I lines
indicates that a He-rich atmospheric composition is required to explain the
line strengths and spectral energy distributions. Our current synthetic
spectra, using an expansion in powers of density to the third order optimised
for brown dwarf atmosphere conditions, significantly underestimate the observed
absorption in the far wings, even predicting smaller total line strength than a
Lorentzian profile. This is due to the handling of multiple perturber
interactions becoming inadequate for the extreme densities of the coolest white
dwarfs. The density expansion would have to be extended at least to the 7th
order for an accurate treatment of such conditions and might break down
altogether in the densest objects. The results of a direct calculation of the
unified profile should therefore be used for model atmospheres of cool
metal-rich white dwarfs. Qualitative comparison of the full adiabatic profile
to the spectrum of WD2356-209 indicates good agreement with the observed line
shape. Observations of the coolest white dwarfs may therefore serve as a
laboratory for testing the physics of the deeper atmospheres and interiors of
brown dwarfs and giant planets.
