[QE-users] Hubbard-U QE vs VASP

[Matteo Cococcioni]

Dear Jhon,

I would be quite surprised if there is a sign-convention mismatch between
the two implementations.
Please keep in mind that, even if you use the same type of pseudootentials,
there is no guarantee that the same U should lead to the
same result (actually this is not the case most often). The value of U
depends on all the details of the calculations and of the pseudopotnetial.
Of course, It would be interesting to compare the two codes on exactly the
same pseudopotential but I am afraid this is not (yet) possible.
If you want to compute it (which I recommend) please consider using the
hp.x code (now part of the QE distribution) that allows you to determine
its value
using a dfpt implementation of linear response theory which is very
efficient and user-friendly (PRB 103, 045141 (2021); PRB98, 085127 (2018)).
Another thing I recommend to use with QE is orthogonalized atomic orbitals
(U_projection_type = 'ortho-atomic') for which the code now has forces and
stresses
(PRB 102, 235159 (2020)).
Please keep in mind also (and pehaps this makes up for most of the
differences you observe) that VASP uses the the projectors of the
seudopotentials  (of PAW type
mostly) to define the atomic occupations then used in the Hubbard
correction. In QE atomic wavefunctions are instead mostly used to project
Kohn-Sham states on.
If you want a stricter comparison between the two perhaps you should use
U_projection_type = 'pseudo' in QE (use PPs containing wavefunctions - see
INPUT_PW.txt
for more details).

Best regards,

Matteo


Il giorno lun 11 ott 2021 alle ore 01:37 Jhon Gonzalez <jhon.gonzalez at usm.cl>
ha scritto:

> Hi,
>
> Perhaps this is a bit off-topic, there is an issue when comparing LDA+U
> results in 2D-materials between Quantum-ESPRESSO and VASP.
> For V2C monolayers, CrI3 mono- and bi-layers, the magnetic stability changes
> and it is impossible to reproduce the results.
>
> For the V2C monolayer with U = 4 eV, while with quantum-ESPRESSO (US and
> PBE), I find that the ground-state
> (GS) solution is FM and the AF solution is about 1 eV above; with VASP U=4
> eV (LDATYPE=2), I find that the GS solution
> is AF and the FM state is 1 eV above (in agreement with
> dx.doi.org/10.1021/jp507336x).
>
> Following the discussion on the implementation of Hubbard's interaction:
> https://lists.quantum-espresso.org/pipermail/users/2020-May/044521.html
> There, it is suggested that in QE the Hubbard-U tends to decrease the gap,
> while in VASP it tends to open it.
> And inspired by the discussion
> http://grandcentral.apam.columbia.edu:5555/tutorials/dft_procedures/linear_response_u/index.html
>
> there they mention an "empirically is a difference in sign convention". I
> tried a VASP calculation with U = -4 eV, and I found an
> FM GS-solution followed by an AF solution 0.3 eV above, following the QE
> stability order.
>
> From my experience comparing QE results with CrI3 experiments, it seems
> that QE uses the proper sign convention for the U term.
> However, I do not have any evidence for this.
>
> I am currently exploring the use of hybrid functionals to address this
> issue, is there an elegant way to settle this dispute?
>
> Best,
>
>
> Jhon W. González
>
> Departamento de Física
> Universidad Técnica Federico Santa María
> VALPARAISO-CHILE
>
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-- 
Matteo Cococcioni
Department of Physics
University of Pavia
Via Bassi 6, I-27100 Pavia, Italy
tel +39-0382-987485
e-mail matteo.cococcioni at unipv.it <lucio.andreani at unipv.it>
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