The equation of state

The equation of state (EOS) is an enlarged and enhanced version of the EOS used in AH95. We include about 500 species (atoms, ions and molecules) in the EOS. This set of EOS species was determined in test calculations. The EOS calculations themselves follow the method discussed in AH95. We did not include dust condensation in the calculations presented here because it does not affect the models in the range of parameters we consider. Models for effective temperatures lower than about 2500-$3000\,{\rm K}$ need to include these effects and will be discussed in a subsequent paper. Some of the molecular data were updated, details are given in the aforementioned paper.