Timrov and Paolo, Thank you very much for your wonderful inputs. I just wanted to clarify a few things that I might have left out in my original message. When I said I used the same parameters for both calculations, this is what I meant: a) Relaxed it in QE ( and then moved to scf etc..). For VASP, I used the relaxed structure to directly perform scf calculation, therefore the structure shouldn't cause any problems between the two. b) Used a well tested (for convergence) k point grid on QE. Used the same k mesh for VASP. c) The same smearing parameters and same hubbard U value ( of 3 eV - for the Chromium atom in my system) for both calculations.
Regarding your final question, yes, I did not compute 'U' from first-principles. It was assumed after referring to various literature about the same material. Best, Shivesh On Wed, May 6, 2020 at 1:59 AM Timrov Iurii <iurii.tim...@epfl.ch> wrote: > Dear Shivesh, > > > > While performing PBE+U calculations on a 2-D material, I wanted to try > using NCPPs and then PAW PPs. For the former, I obviously tried QE and for > the latter, VASP. > > > In Quantum ESPRESSO, DFT+U can be used with any type of pseudopotentials > (NC PPs, US PPs, PAW PPs). So your motivation to switch to VASP because of > PAW PPs looks strange to me. > > > However, it is important to note that DFT+U for PAW PPs in QE and VASP is > not implemented in the same way. There are some technical differences (in > QE the projections are done on atomic/ortho-atomic orbitals while in VASP > on \beta projector functions). This aspect will be discussed briefly in the > paper which we are working on (Timrov, Marzari, Cococcioni). > > > > The electronic bandstructures look very similar (the parameters are > completely the same, except for plane wave cutoffs, as you might imagine) > but the band gap is different. > > > Which parameters are completely the same? Do you refer to a k points > sampling, smearing, lattice parameter? Or Hubbard U? > > > > With NCPPs, the Hubbard 'U' seems to suppress the band gap (compared to > U=0) whereas for VASP calculation, it seems to increase the band gap > (compared to U=0). Is there a qualitative explanation for this? > > > Which values of U do you use in the two codes? How did you choose them? > > > Please note that *the Hubbard U parameter is not a universal parameter*. > If you compute it from first principles (e.g. using linear response theory, > cRPA, or other approach), its value depends on many factors (type of the > Hubbard manifold, pseudopotentials, functional, oxidation state, etc.). So > if you use the same value of U in two different codes (i.e. different > implementations of DFT+U), with different PPs, with different Hubbard > manifolds, etc., then the effect of the Hubbard correction with same U will > be different. > > > The correct procedure is the following: to compute Hubbard parameters from > first-principles for a given set of computational parameters (Hubbard > manifold, PPs, functional, etc.), and *use it consistently with exactly > the same set of parameters*. In QE, Hubbard parameters can be computed > using the HP code. Hubbard U is not global, and hence it is not portable > (i.e. you cannot compute it in one code with one set of parameters, and > then use it in another code with another set of parameters; even in the > same code you cannot compute with one set of parameters and then use it in > the same code with another set of parameters). > > > Greetings, > > Iurii > > > -- > Dr. Iurii Timrov > Postdoctoral Researcher > STI - IMX - THEOS and NCCR - MARVEL > Swiss Federal Institute of Technology Lausanne (EPFL) > CH-1015 Lausanne, Switzerland > +41 21 69 34 881 > http://people.epfl.ch/265334 > ------------------------------ > *From:* users <users-boun...@lists.quantum-espresso.org> on behalf of > Shivesh Sivakumar <shiveshsivaku...@gmail.com> > *Sent:* Wednesday, May 6, 2020 12:35:30 AM > *To:* Quantum ESPRESSO users Forum > *Subject:* [QE-users] Different implementations of the Hubbard 'U' > between QE and VASP > > Hello all, > > While performing PBE+U calculations on a 2-D material, I wanted to try > using NCPPs and then PAW PPs. For the former, I obviously tried QE and for > the latter, VASP. The electronic bandstructures look very similar (the > parameters are completely the same, except for plane wave cutoffs, as you > might imagine) but the band gap is different. > With NCPPs, the Hubbard 'U' seems to suppress the band gap (compared to > U=0) whereas for VASP calculation, it seems to increase the band gap > (compared to U=0). Is there a qualitative explanation for this? > > I am very sorry if this question is not totally appropriate for the forum > but I just want to know if conceptually, there are some differences in > implementation that I am missing out. > > Best, > Shivesh Sivakumar > University of Washington > Seattle, WA-98105 > _______________________________________________ > Quantum ESPRESSO is supported by MaX (www.max-centre.eu/quantum-espresso) > users mailing list users@lists.quantum-espresso.org > https://lists.quantum-espresso.org/mailman/listinfo/users
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