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

[Shivesh Sivakumar]

Nicola,

Sorry to bring up the subject again but I still can't reconcile something.
I understand that 'U' is not universal and not completely portable between
different PPs. But what I find odd is the effect of 'U'. On QE (with NCPP),
'U' seems to suppress the band gap whereas in VASP (with PAW), 'U' seems to
widen the band gap. This is what looked very strange.

Best,
Shivesh

On Wed, May 6, 2020 at 10:22 AM Shivesh Sivakumar <
shiveshsivakumar at gmail.com> wrote:

> 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
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20200506/a4fff1ab/attachment.html>