[Wien] band gap for structure with a defect

[Igor I Mazin]

1. Undoped BaTiO3 is an insulator. An oxygen vacancy introduces electron 
doping. When you dope an insulator, you get a metal. In a metal, the gap 
is zero.
2. U=15 eV is an insanely large and unphysical value. You are better off 
with no U at all. For small doping (small concentration of vacancies) 
Hubbard correlations are irrelevant.
3. 2x2x1 with one vacancy is doping of 0.5 e per formula unit. This is 
unphysically large (like, by three orders of magnitude) and I would not 
trust a single number from this simulation. Check 
https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.5.033080

On 9/13/2023 13:19, Natalia Andreeva wrote:
> Dear WIEN2k users,
> 
> I am calculating the band gap of bulk BaTiO3 using the LDA+U with PBE 
> functional. For the values below, the band gap was 2.002 eV (which I can 
> explain by the choice of the PBE functional). When I moved to a 
> supercell with a defect (oxygen vacancy, supercell dimensions 2x2x1), 
> the band gap became 0.0 eV.
> I tried to increase the Hubbard parameter U. For example, for bulk 
> calculations with U=1.121 Ry = 15.252 eV, the band gap was 3.391 eV. 
> However, running with the same parameters for a supercell with a defect 
> gave a calculated value of 0.889 eV.
> I have the following questions:
> 1. Why does the band gap decrease so much when going from bulk to 
> supercell with a defect?
> 2. Is it worth changing the PBE functional to another one if the band 
> gap for bulk is less than the experimental one?
> 3. If I continue the calculations on PBE+U, how can I improve the values?
> 
> With Best Regards,
> Natalia
> 
> 
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-- 
Igor Mazin, Prof. of Advanced Studies
Quantum Science and Engineering Center
Department of Physics and Astronomy
George Mason University
phone 1-703-503-8152 (h)
http://mason.gmu.edu/~imazin2