[Wien] band gap for structure with a defect

[Natalia Andreeva]

Thank you very much

On Wed, Sep 13, 2023 at 8:32 PM Igor I Mazin <imazin2 at gmu.edu> wrote:

> 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
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-- 
Best Regards,
Natalia Andreeva.
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