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TiO and H$_2$O absorption lines in cool stellar atmospheres

We compare the structures of model atmospheres and synthetic spectra calculated using different line lists for TiO and water vapor. We discuss the effects of different line list combinations on the model strutures and spectra for both dwarf and giant stars. It is shown that recent improvements result in significantly improved spectra, in particular in the optical where TiO bands are important. The water vapor dominated near-IR region remains problematic as the current water line lists do not yet completely reproduce the shapes of the observed spectra. We find that the AMES TiO list provides more opacity in most bands and that the new, smaller oscillator strengths lead to systematically cooler temperatures for early type M dwarfs than previous models. These effects combine and will help to siginificantly improve the fits of models to observations in the optical as well as result in improved synthetic photometry of M stars. We show that the Davis 1986 \fel-values for the $δ$ and $ϕ$ bands of TiO best reproduce the observed (V-I) color indices.