[Wien] mBJ for FeO

[John McLeod]

Hello all,

I tried to calculate the electronic structure of FeO using mBJ.

   1. I took the cubic FeO structure, made a 2x2x2 primitive supercell, 
relabeled the Fe sites "1" and "2" to get the appropriate AFM ordering, 
ran sgroup, and obtained a rhombohedral cell with 2 Fe sites and 1 O 
site. This structure looks correct when viewed with VESTA or xcrysden, 
so I think I am using the correct structure.

   2. I run a spin-polarized PBE calculation, initializing the Fe to 
"up" and "down", and the O to "no spin", this converges quickly and 
obtains a zero band gap as expected.

   3. I run an mBJ calculation using PRATT mixing, slowly increasing the 
mixing factor. The energy converges in 20 cycles or so, however the 
charge never converges - it keeps "sloshing" back and forth between the 
two Fe sites. The band gap for this system remains at zero.

In PRL 102 226401 (2009) and mBJ calculation on FeO reports a gap of 
1.82 eV. May I ask Dr. Fabian Tran and/or Dr. Peter Blaha, to obtain 
these results did you:
    1. Perform an AFM calculation or just a spin-polarized calculation?
    2. Perform any sort of structural optimization?
    3. Use PRATT mixing only, or switch back to MSR1?

I also tried this approach with hematite (Fe2O3), obtaining a 
rhombohedral structure with 4 inequivalent Fe sites (as expected), and 
following the same steps I obtained good charge and energy convergence 
in mBJ as well as a reasonably accurate band gap - so I find it a bit 
curious that my approach fails for FeO.

I would greatly appreciate a quick tip on how the original calculations 
in PRL 102 226401 were performed.

Regards,
John McLeod