Opacities are of vital importance in astrophysics necessary to model stellar atmospheres and interiors, and below about 6000 K diatomic molecules make an increasing contribution to that opacity. Moreover, at around 3000 K polyatomic molecules, such as H2O for an oxygen rich solar composition, start playing an important rôle, and additionally at high pressures collision induced H2 – H2 and H2 – He absorption is important for late-type main sequence stars.

In order to investigate the importance of molecular opacities, in Sharp (1992) we computed and compared opacities for a solar and a three times enhanced CNO abundance, and some further comparisons were made in Sharp et al. (1992). Some of these results are briefly summarised here.