[QE-users] hp.x code overestimates Hubbard U value

[Timrov Iurii]

Dear Soumyadeep,


Hubbard U is not a universal parameter. If you compute Hubbard U from first principles, e.g. using linear response theory (LRT) [PRB 71, 035105 (2005); PRB 98, 085127 (2018)] then the value of U depends on many factors:

- Manifold of localized functions (atomic, ortho-atomic, Wannier, etc.);

- Pseudopotentials;

- Oxidation state;

- Exchange-correlation functional (LDA, PBE, PBEsol, etc.);

- Environment (e.g. U for Mn-3d in MnO and BiMnO3 is not exactly the same).


For example, check the paper by H. Kulik and N. Marzari, J. Chem. Phys. 129, 134314 (2008). In particular, check the appendix in this paper, and you will see that U is very sensitive to the oxidation state.


Another aspect is whether U was computed in "one-shot" or "self-consistently". If you want to learn more about the self-consistent U, I recommend these papers: H. Hsu et al., PRB 79, 125124 (2009); C. Ricca et al., PRB 99, 094102 (2019); M. Cococcioni and N. Marzari, PRM 3, 033801 (2019).


> ..."U" lies between 3.0-3.5 eV...


Where do these values come from? Are they empirical? Or were they computed with cRPA? Please note that  comparing U is not very meaningful (in my understanding), especially if you do not specify how it was determined. As I wrote above, the value of U computed from first principles depends on many factors. So if you compute it using different methods (LRT vs cRPA), different localized functions (atomic vs Wannier), different oxidation states, etc. then you will not get the same U. What is important is to use U (for calculations of band structure, DOS, phonons, etc.) with the same parameters which which it was computed.


I recommend to have a look at the hands-on about calculation of U using Quantum ESPRESSO (hp.x code):

https://agenda.ethernet.edu.et/event/33/


Hope this helps! If something is not precise in my explanations, it would great if other experts correct me.


Best regards,

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 Soumyadeep <soumyadeep at rrcat.gov.in>
Sent: Wednesday, September 11, 2019 4:35:37 PM
To: Users
Subject: [QE-users] hp.x code overestimates Hubbard U value

Dear QE users,

   I am getting a overestimated value of Hubbard parameters using hp.x
executable of Quantum Espresso. My material is iron based
superconductors (LiFeAs) for which "U" lies between 3.0-3.5 eV, but
using this code I get it around 6.4-6.5 eV.

   Can anyone find out possible mistakes?

with many thanks and best regards
Soumyadeep
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Soumyadeep Ghosh,
Senior Research Fellow,
Homi Bhabha National Institute (HBNI),
Raja Ramanna Centre for Advanced Technology, Indore, India-452013
Mob: (+91)9424664553
User Lab: 0731244-2580
Email: soumyadeepghosh35 at gmail.com, soumyadeep at rrcat.gov.in
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