[Wien] Properties of corresponding atoms in afm calculations

[Peter Blaha]

The forces are calculated from 4 contributions.

:FHFxxx (coulomb forces) are calculated in lapw0 for each atom
individually. I guess here is the source of your (very small) differences.
There is NO symmetry constraint on these numbers and my guess is, that you 
did not start with an exactly antiferromagnetic startup ? (Setting all 
moments to ZERO in case.inst, except for the AFM atoms)?
How did FHFxxx look during scf-cycles ?

:FCOxxx  Core forces: Also these numbers are calculated without
symmetry constraint and it might be that the potential inside the
atoms is not 100% identical.

:FVAxxx and :FSUxxx : these are the IBS corrections calculated in lapw2
and these numbers should have the "symmetry constraint". Usually they are
added only in the last scf-iteration.

Please check already for the first scf iteration (starting with dstart)
if the force contributions show differences (and if they become smaller 
during scf, because you mix in a "true" AFM density). 


> >What does "different forces" mean ? Could that be "numerical noise"
> >(like forces of +20 and +21), or is it more like +20 and +10 ??
> >
> >  
> >
> You may call it "numerical noise" (it's like 10.276 vs. 10.400 etc.). 
> But in my opinion this must not happen in an afm calculation since the 
> down electrons are identical to the up electrons (by clmcopy). Thus 
> everything that happens on atom A must be _exactly_ the same on atom A' 
> (maybe with different sign, but same absolute value).
> The different forces are quite inconvenient when doing a minimization as 
> pointed out before.
> 
> >In the "noise" case, your procedure should be fine (eventually you may need
> >better scf-convergence ?), otherwise something more
> >serious may happen.
> >
> >  
> >
> I think the scf-convergence is okay (-ec 0.00001 -cc 0.00001). By the 
> way this has no influence on the afm argument that properties of 
> corresponding atoms should be the same.

See my arguments above.

                                      P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
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