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
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