[Wien] Problems with occupancy

[Gerhard Fecher]

No
you may need to use a programm that is capable to use Coherent Potential Approximation (CPA)
e.g. (FPLO, KKR or some LMTO) (but you loose some nearest neighbour interactions if non-local CPA is not implemented)
the possibilities you have with Wien2k are
a) use supercells (you have an artificial symmetry that is absent in "real" alloys with "non-integer" site occupation, say better random disorder (see at end))
and average over all inequivalent occupations of the sites but this may need large number of calculations for large supercells.
b) with some restrictions Virtual Crystal Approximation (as a) and there is no real relation to the atoms of the periodic table and may only work for neighboring atoms)
by replacing an atom with Z by one with Z+-epsilon valence electrons/core charge

Anyway, in nature you will never have two different atoms occupying the same site (how should that be possible, and fractional atoms don't exist)
it is just a way how to describe a X-ray diffraction pattern when one has disorder on the sites of a regular lattice, that is it is
also only some type of averaging as in the ab initio methods given above.

Check the mailing list, you are not the first one asking that question.

Ciao
Gerhard

====================================
Dr. Gerhard H. Fecher
Institut of Inorganic and Analytical Chemistry
Johannes Gutenberg - University
55099 Mainz
________________________________________
Von: wien-bounces at zeus.theochem.tuwien.ac.at [wien-bounces at zeus.theochem.tuwien.ac.at]" im Auftrag von "Che Franklin N. [c89vd at unb.ca]
Gesendet: Dienstag, 27. September 2011 21:47
Bis: wien at zeus.theochem.tuwien.ac.at
Betreff: [Wien] Problems with occupancy

Dear Wien Users
  I have a problem. I was wondering if there is a way to account for
occupancy (i.e
  when two atoms occupy the same positions in Wien?
  Any help would be appreciated.
  Thanks

  Franklin
  Chemistry Dep't UNB


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