[Wien] Force optimization of a spinel compound

[Laurence Marks]

This is not a simple question to answer. In both DFT and XRD the symmetry
of the structure is an input parameter, and in both cases it is sometimes
wrong. The most obvious question to ask is whether you used the same Space
Group for your refinement in Wien2k that was used in the XRD refinement. I
have a suspicion that you did not, as the presence of (x,x,x) positions
implies a Space Group constraint and if you are getting (x,y,z) then you
probably have a different symmetry in Wien2k.

Check first that the symmetry is completely the same.

If they are the same, the question then would be why one gets (x,x,x) in
the XRD refinement? It could be disorder or something else, but without
more information it is hard to guess.


On Mon, Sep 8, 2014 at 11:37 AM, shamik chakrabarti <shamikphy at gmail.com>
wrote:

>  Dear wien2k users,
>
>            We have optimized atomic coordinates of a spinel cubic
> compound containing 56 atoms/cell. The most important variation we have
> seen between experimental structural coordinates & DFT (GGA+U) optimized
> coordinates is that.
>
>  the oxygen coordinate changes from (x,x,x) configuration to (x,y,z)
> configuration as we have not constrained the oxygen coordinates during
> optimization.
>
>  Although, this change has generated a structure having very small forces
> (< 1 mRy/Bohr) per atoms, the change in oxygen coordinates are significant
> (difference between x & z is 0.021) enough to change the relative
> intensities of different peaks of XRD patterns.
>
>  Now my query is (before starting the experimental simulation by myself
> again!!) whether there is a way to constrain atomic coordinates such that
> it remain constrained to (x,x,x) configuration while changing the value of
> x?
>
>  & also whether such a constrained way should be adopted or should we
> accept the DFT optimized structure (a bit different from experimental
> one!!) as it is for further calculation?
>
>  Looking forward to your opinions & suggestions.
>
>  Thanks in advance,
>
>  with regards,
>
> --
> Shamik Chakrabarti
> Senior Research Fellow
> Dept. of Physics & Meteorology
> Material Processing & Solid State Ionics Lab
> IIT Kharagpur
> Kharagpur 721302
> INDIA
>



-- 
Professor Laurence Marks
Department of Materials Science and Engineering
Northwestern University
www.numis.northwestern.edu
Corrosion in 4D: MURI4D.numis.northwestern.edu
Co-Editor, Acta Cryst A
"Research is to see what everybody else has seen, and to think what nobody
else has thought"
Albert Szent-Gyorgi
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