[QE-users] Hubbard-U QE vs VASP

[Iurii TIMROV]

Dear All,


In addition to what Matteo explained, I would like to point your attention to the following article which is relevant (in my opinion) for this discussion (QE vs VASP): https://aip.scitation.org/doi/10.1063/1.4945608


Greetings,

Iurii


--
Dr. Iurii TIMROV
Senior Research Scientist
Theory and Simulation of Materials (THEOS)
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 Matteo Cococcioni <matteo.cococcioni at unipv.it>
Sent: Monday, October 11, 2021 10:22:08 AM
To: Quantum ESPRESSO users Forum
Subject: Re: [QE-users] Hubbard-U QE vs VASP

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<mailto: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<http://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

_______________________________________________
Quantum ESPRESSO is supported by MaX (www.max-centre.eu<http://www.max-centre.eu>)
users mailing list users at lists.quantum-espresso.org<mailto:users at lists.quantum-espresso.org>
https://lists.quantum-espresso.org/mailman/listinfo/users


--
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<mailto:lucio.andreani at unipv.it>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20211011/9a27882e/attachment.html>