[Wien] Properties of corresponding atoms in afm calculations

[Ulrich Keßler]

Dear Prof. Blaha,

thank you for your answer.

I think I did not describe my problem well enough.

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

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
>>http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
>>
>>    
>>
>
>
>                                      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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