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

[Paolo Giannozzi]

Iurii is perfectly right: U is not a portable parameter. This being said,
it seems to me strange that two calculations performed with different
approaches but with the same value of U may yield opposite results. It
seems to me more likely that one of the two calculations is not correctly
set up.

Paolo


On Wed, May 6, 2020 at 10:59 AM Timrov Iurii <iurii.timrov at 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-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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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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