Initially, spectral line opacities were economically treated by the ODF approximation, but later the more flexible and realistic opacity sampling scheme has been adopted. In the OS scheme, line opacities are directly tabulated for a large number of wavelength points (10$^{5}$) as a function of temperature and pressure.

The shift in the MARCS code from using ODFs to the OS scheme avoided the sometimes unrealistic assumption that the line opacities of certain relative strengths within each ODF wavelength interval overlap in wavelength irrespective of depth in the stellar atmosphere. This assumption was found to lead to systematically erroneous models, in particular when plolyatomic molecules add important opacities to surface layers [@ekberg01]. The current version of the MARCS code used for the present project and for the more extensive MARCS model atmosphere data base[^2] was presented and described in detail by @gustafsson02. The model atmospheres presented in this paper adds large variaty in \[C/M\] and \[$\alpha$/M\] abundances to the already existing grids by covering these abundances systematically from -1 to +1 for each metallicity.

Our main purpose is to update the previous ATLAS9 grid and publish new MARCS models to provide a large composition range to use in the APOGEE survey 