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

[Shivesh Sivakumar]

Nicola,

Thank you very much for your valuable input. It definitely makes everything
more clear to me.

Best,
Shivesh

On Wed, May 6, 2020 at 9:50 AM Nicola Marzari <nicola.marzari at epfl.ch>
wrote:

>
>
>
> Dear Shivesh,
>
>
> this is really important: as Iurii and Paolo already mentioned, U is
> *not* a universal parameter. For the same element and for the same
> material, it can be easily 3eV or 7eV (e.g. if the Hubbard manifold it
> acts upon has been taken say from a pseudopotential (NCPP/USPP/PAW)
> generated in a neutral configuration or in an oxidized one).
>
> So, that means that the same identical material - say, MnO - should be
> studied using a U for Mn of 3.13 if you use pseudopotential "A", and
> 7.56eV when you use pseudopotential "B" for Mn. For a slightly
> compressed unit cell, maybe the U for "A" would be 3.10eV, and for "B"
> 7.74eV.
>
> Looking at the literature wouldn't help - especially (IMNSHO) because
> almost noone has understood what the U is.
>
>                         nicola
>
>
>
> On 06/05/2020 18:25, Shivesh Sivakumar wrote:
> > 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
> --
> ----------------------------------------------------------------------
> Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
> Director, National Centre for Competence in Research NCCR MARVEL, EPFL
> http://theossrv1.epfl.ch/Main/Contact http://nccr-marvel.ch/en/project
>
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