[QE-users] Different implementations of the Hubbard 'U' between QE and VASP

[Timrov Iurii]

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-bounces at lists.quantum-espresso.org> on behalf of Shivesh Sivakumar <shiveshsivakumar at 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
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