ATLAS12 computes model atmospheres for individual abundances. 
The line opacity is computed with the Opacity Sampling (OS) method.
Once a model is computed, the corresponding flux can be obtained 
with SYNTHE. In this case:

The ATLAS12 ouput model, for example ap04t4970g46k1at12.dat, MUST BE MODIFIED 
in  ap04t4970g46k1at12.mod:

1) ADD the following control cards at the top of the ATLAS12 model 
(this holds also for an ATLAS9 input model):

SURFACE INTENSI 17 1.,.9,.8,.7,.6,.5,.4,.3,.25,.2,.15,.125,.1,.075,.05,.025,.01
ITERATIONS 1 PRINT 2 PUNCH 2
CORRECTION OFF
PRESSURE OFF
READ MOLECULES
MOLECULES ON

2) DROP the cards of the ATLAS12 model:
                                   ABUNDANCE TABLE
                                   1H  XXX  2He XXX
                                   3Li XXX XX 4Be XXX XX etc..
                                   up to
                                   98Cf XXX CC  99Es XXX XX