[Wien] Optimization of an spinel structure

[Fecher, Gerhard]

In this particular case, if your x parameter is other then 1/4 then you are not longer in space group 227 already in the non magnetic case,
your Wyckoff positions in SG 227 are 16-fold for A and 8 -fold for both B and X degenerate
now if you put x = 1/4 + delta, then the site for the X atoms wouldl be 32-fold degenerate (in SG 227)

It seems your structure is ill defined from the beginning, you need to use the correct spave group.

It seems you just try to use the highest symmetry for a non-magnetic case to model the antiferromagnetic state, this is doubtful.
you need to test also other arrangements, check the possible magnetic space groups.
Just to mention, if you have a frustrated system with non-collinear spins, then you may not just set them up and down
you can do it, but what do you learn ? 



Ciao
Gerhard

DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:
"I think the problem, to be quite honest with you,
is that you have never actually known what the question is."

====================================
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