  Dense, He-rich atmospheres of cool white dwarfs represent a challenge to the
modeling. This is because these atmospheres are constituted of a dense fluid in
which strong multi-atomic interactions determine their physics and chemistry.
Therefore, the ideal-gas-based description of absorption is no longer adequate,
which makes the opacities of these atmospheres difficult to model. This is
illustrated with severe problems in fitting the spectra of cool, He-rich stars.
Good description of the infrared (IR) opacity is essential for proper
assignment of the atmospheric parameters of these stars. Using methods of
computational quantum chemistry we simulate the IR absorption of dense He/H
media. We found a significant IR absorption from He atoms (He-He-He CIA
opacity) and a strong pressure distortion of the H$_2$-He collision-induced
absorption (CIA). We discuss the implication of these results for
interpretation of the spectra of cool stars.
