ATMOS integrates the hydrostatic equation from a given temperature structure and chemical composition. The result is the pressure stratification with continuous opacities and particle concentrations. The result can then be used as input to LINFOR.
There is an alternative version NATM which will not calculate
the pressures but reads them from NPTAU.DAT
instead. All that NATM does is to compute particle concentrations
and continuous opacitities. This version is useful if the temperature
structure has been computed with another stellar atmospheres program
such as ATLAS9 and should be fed into LINFOR.
One has to make sure, however, that the chemical mix is identical in
ATMOS and the other program.