[Wien] Properties of corresponding atoms in afm calculations

[Laurence Marks]

Which version are you running? Numerical errors can creep in which get
inflated in mini because it (correctly) will diagnose small symmetry
breaking as indicating a lower symmetry. Many, but not all of these
have been trapped in the later versions.

On 9/11/06, Ulrich Keßler <ukessler at uni-bonn.de> wrote:
> 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
> >>_______________________________________________
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> >>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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> >
> >
> >
> >
>
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-- 
Laurence Marks
Department of Materials Science and Engineering
MSE Rm 2036 Cook Hall
2220 N Campus Drive
Northwestern University
Evanston, IL 60208, USA
Tel: (847) 491-3996 Fax: (847) 491-7820
email: L-marks at northwestern dot edu
http://www.numis.northwestern.edu