  Reliable modeling of the atmospheres of cool white dwarfs is crucial for
understanding the atmospheric evolution of these stars and for accurate white
dwarfs cosmochronology. Over the last decade {\it ab initio} modeling entered
many research fields and has been successful in predicting properties of
various materials under extreme conditions. In many cases the investigated
physical regimes are difficult or even impossible to access by experimental
methods, and first principles quantum mechanical calculations are the only
tools available for investigation. Using modern methods of computational
chemistry and physics we investigate the atmospheres of helium-rich, old white
dwarfs. Such atmospheres reach extreme, fluid like densities (up to grams per
cm$^3$) and represent an excellent laboratory for high temperature and pressure
physics and chemistry. We show our results for the stability and opacity of
$\rm H^-$ and $\rm C_2$ in dense helium and the implications of our work for
understanding cool white dwarfs.
