[Wien] Optimization of an spinel structure

[Fecher, Gerhard]

Sorry,
it depends on the setting for space group 227 whether X=1/4 results in a  8- or 32-fold degenerate position, I did not check which one Wien2k uses (unfortunately one can not use crystallographic Wykoff positions directly).
if you have already the 32-fold degenerate site  (I guess wien2k reports a smaller cell with 8 sites)  then min should find the correct positions by minimizing the forces.

The remarks on the magnetic order are not touched by that.

Ciao
Gerhard

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====================================
Dr. Gerhard H. Fecher
Institut of Physics
Johannes Gutenberg - University
55099 Mainz
________________________________________
Von: Wien [wien-bounces at zeus.theochem.tuwien.ac.at] im Auftrag von delamora [delamora at unam.mx]
Gesendet: Dienstag, 2. November 2021 04:49
An: wien at zeus.theochem.tuwien.ac.at
Betreff: [Wien] Optimization of an spinel structure

Dear WIEN community,
I am trying to optimize a spinel structure.
The normal spinel AB2X4 is FCC with SG 227
A: 1/8,1/8,1/8
B: 1/2,1/2,1/2
X: x,x,x  x~1/4
so it has 2 free parameters; 'volume' and 'x,x,x'
but in my cases B is magnetic and it has antiferromagnetic coupling, B also forms the 3D Kagome structure so it is geometrically frustrated.
The volume optimization can be easily done with option 'optimize'
but for the 'x' parameter there is a problem;
To treat the magnetism the best option, in my opinion, is to put half of the B atoms 'up' and the other half 'dn', there are 4 in the primitive cell, so 2 are 'up' and 2 'dn'.
With this, then the FCC cell becomes BCC (SG 74 Imma) and the 8 'X' atoms that were in one wyckoff position are now in two wyckoff positions with 4 atoms in each.
So, if the forces are minimized, 'runsp -min', will move these two 'X' groups in directions that will break the original cubic symmetry.
One posibility is to make 4 'struct' files, in the original SG 227, with slightly different 'x' positions, for example, the initial x,x,x is;
x= 0.254, so I make 4 struct files with; 0.251, 0.253, 0.255, 0.257
after this one should make them antiferromagnetic (SG 74 Imma) then run them separatelly, then one can obtain the optimum 'x' parameter.

I tried to use a modified 'optimize.job' but it did not work.
I also tried to run these 4 'struct' files in the same directory but I found that when I put the next case.struct into the directory, I had to initiate the system in order to be able to run them.

In other words, if I run the system with 1-struct, then for the next 2-struct I cannot just change 1-struct for 2-struct and run.
For the case of changes of volume then the optimize.job works fine, but for small changes in 'x' it does not work.

I anybody has an idea that would make this case simpler I would apreciate.
Thank you anyway

Pablo