Re: [Wien] band gap for structure with a defect
Thanks a lot On Wed, Sep 13, 2023 at 8:56 PM Peter Blaha wrote: > In addition to what Igor said: > > If you want a reasonable gap for the bulk, you can use mBJ+U (with a > small U of a few eV). > > Still, for your cell, you won't get an insulator (also doped Si is > "metallic" and conducting, otherwise a computer would not work ...). > > PS: Draw the DOS and compare with the undoped DOS. Maybe you understand > then better. > > > Am 13.09.2023 um 19:19 schrieb 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 > > > > > > ___ > > 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 > > -- > --- > Peter Blaha, Inst. f. Materials Chemistry, TU Vienna, A-1060 Vienna > Phone: +43-158801165300 > Email: peter.bl...@tuwien.ac.at > WWW: http://www.imc.tuwien.ac.at WIEN2k: http://www.wien2k.at > - > > ___ > 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 > -- Best Regards, Natalia Andreeva. ___ 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
Re: [Wien] band gap for structure with a defect
Thank you very much On Wed, Sep 13, 2023 at 8:32 PM Igor I Mazin 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 > > > > > > ___ > > 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 > -- Best Regards, Natalia Andreeva. ___ 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
Re: [Wien] band gap for structure with a defect
In addition to what Igor said: If you want a reasonable gap for the bulk, you can use mBJ+U (with a small U of a few eV). Still, for your cell, you won't get an insulator (also doped Si is "metallic" and conducting, otherwise a computer would not work ...). PS: Draw the DOS and compare with the undoped DOS. Maybe you understand then better. Am 13.09.2023 um 19:19 schrieb 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 ___ 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 -- --- Peter Blaha, Inst. f. Materials Chemistry, TU Vienna, A-1060 Vienna Phone: +43-158801165300 Email: peter.bl...@tuwien.ac.at WWW: http://www.imc.tuwien.ac.at WIEN2k: http://www.wien2k.at - ___ 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
Re: [Wien] band gap for structure with a defect
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
