[Wien] Properties of corresponding atoms in afm calculations

[Peter Blaha]

Hi,

> runafm seems not to be the problem. After an afm-cycle runsp does not 
> change anything significantly.

What about the forces after runsp ? Do they become "equivalent" or stay
different as obtained from runafm ?

What does "different forces" mean ? Could that be "numerical noise"
(like forces of +20 and +21), or is it more like +20 and +10 ??

In the "noise" case, your procedure should be fine (eventually you may need
better scf-convergence ?), otherwise something more
serious may happen.

Please, also check carefully if the moments are really AF, or if you run
into an ferrimagnetic solution.

A typical situation of the latter would be "bcc Mn", which is NOT
antiferromagnetic, but the two Mn atoms get a slightly different moment.
In such a situation runafm and runsp will naturally lead to differences. 



> 
> My problem is that the forces of the corresponding atoms are not the 
> same. During a minimization cycle this leads to different shifts of the 
> corresponding atoms so that they are not any longer symmetry equivalent 
> in the ferromagnetic supergroup. At the moment I stop the miminization 
> after some cycles and edit the new struct file so that corresponding 
> atoms possess equivalent sites. In my opinion the equivalent atoms in 
> the supercell should have the same forces and should be shifted by the 
> same amount.
> 
> This is what I did:
> I constructed the afm supercell in P1121/c (SG 14) containing 12 
> independent atoms from the ferromagnetic structure in the supergroup 
> B112/b (SG 15, 6 independent atoms). Since this is a klassegleiche 
> subgroup the unit cell dimensions did not change. Every two atoms of  
> the afm structure in P21/c correspond and possess positions which are 
> equivalent in C2/c. During afminput I had to give the symmetry relation 
> between these corresponding atoms (simply 0.5, 0, 0.5 since the B 
> centering vanishes). After that runafm worked without problems except 
> for the different forces of the corresponding atoms (I did not check 
> other atom dependent properties).
> 
> Is this something to live with (some kind of rounding error) or 
> something more serious? And does an afm calculatiom make sense when the 
> corresponding atoms are not mapped onto each other exactly by 
> case.inclmcopy?
> 
> If you need more information (struct-files etc.) please let me know.
> 
> Thank you for your help,
> Ulrich Keßler
> 
> Peter Blaha wrote:
> 
> >runafm and runsp should yield the same results, in particular when 
> >continuing with runsp AFTER a runafm convergence results must be 
> >identical. (The could be cases when 
> >runsp does not find and afm state, but eg. 2 moments which are different 
> >(ferrimagnetic),..)
> >
> >Maybe you have a problem with your struct file (symmetry) or with afminput 
> >(case.inclmcopy ?) ?
> > 
> >
> >
> >  
> >
> >>when calculating an antiferromagnetic compound using the runafm_lapw 
> >>script, the symmetry relations between the antiferromagnetic atoms is 
> >>exploited (they share the same site in the ferromagnetic supergroup). 
> >>Thus I would expect that all properties of the corresponding atoms 
> >>should be the same, especially the forces. However, I find that the 
> >>forces of corresponding atoms tend to diverge in the course of an 
> >>afm-SCF-cycle (not much, but significantly). Even more so when 
> >>performing a minimization of positions.
> >>
> >>Is the reason some rounding error?
> >>Is this serious? If yes, what could be done about it?
> >>
> >>Any comments are appreciated. Thank you.
> >>
> >>Ulrich Keßler
> >>
> >>
> >>Dr. Ulrich Keßler
> >>Institut für Anorganische Chemie
> >>Universität Bonn
> >>Gerhard-Domagk-Str. 1
> >>D-53121 Bonn
> >>Germany
> >>Tel.: (+49) (0)228/73-5334
> >>Fax: (+49) (0)228/73-5660
> >>_______________________________________________
> >>Wien mailing list
> >>Wien at zeus.theochem.tuwien.ac.at
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> >>
> >>    
> >>
> >
> >
> >                                      P.Blaha
> >--------------------------------------------------------------------------
> >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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> >
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                                      P.Blaha
--------------------------------------------------------------------------
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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