[Wien] band gap for structure with a defect

[Natalia Andreeva]

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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