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 > > > _______________________________________________ > Wien mailing list > Wien@zeus.theochem.tuwien.ac.at > http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien > SEARCH the MAILING-LIST at: > http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html -- 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 _______________________________________________ Wien mailing list Wien@zeus.theochem.tuwien.ac.at http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien SEARCH the MAILING-LIST at: http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html