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.
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