[Wien] mBJ for FeO

[tran at theochem.tuwien.ac.at]

With strongly correlated solids, it is usually possible to
stabilize several electronic configurations. For FeO, if
you start a mBJ or LDA+U calculation from the PBE density,
then a metallic state is obtained. The state with a gap, which
is more stable, can be obtained by running first a constrained
LDA+U calculation with -orbc:

1) change manually the occupation (that you need to know) in
case.dmatup/dn of a LDA+U calculation,
2) execute "x orb -up/dn" to generate case.vorbup/dn
3) run LDA+U with -orbc instead of -orb
4) save the contrained calculation when it is finished
4) run LDA+U or mBJ as usual.

The struct and dmat files of a recent LDA+U calculation on FeO are
attached.

F. Tran

On Thu, 10 Dec 2015, John McLeod wrote:

> 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
>
>
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