differences must be related to the updated solar chemical composition, as the Castelli-Kurucz models are also computed with a corrected H$_{2}$O linelist. The most significant change in the solar composition corresponds to the reduction in oxygen, nitrogen and carbon abundances, all of which decrease in the update. This causes a reduction in the Rosseland opacity at a given temperature and gas pressure. The decreased Rosseland opacity leads to a subsequent increase in the total pressure, which is only partially compensated by a reduction in the electron density.

@gustafsson02 present some further comparisons between MARCS models and ATLAS9 models of @castelli01. Other comparisons between MARCS, ATLAS9, and PHOENIX model atmospheres and spectra were discussed by @plez02.

![Examples of how different \[C/M\] and \[$\alpha$/M\] content changes the temperature profile in the ATLAS9 and MARCS atmospheres for T$_{\rm eff}$ = 4000K, log $g$ = 1. This figure shows the temperature ($T$), gas pressure ($P$), electron number density ($N_{\rm e}$) and the relative difference (where 0.5 corresponds to 50%, 1 corresponds to 100%) of these parameters between \[C/M\]=\[$\alpha$/M\] = 0 and \[C/M\]=\[$\alpha$/M\] = $-$1, and 1 as a function of optical depth (log $\tau$). The symbols $T_{0}$, $P_{0}$, $N_{e, 0}$ correspond to \[C/M\]=\[$\alpha$/M\] = 