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

[Soumyadeep]

Dear Iurii,
   Thanks you very much for the detailed explanation.

   >> ..."U" lies between 3.0-3.5 eV...
   The determined value of U is calculated using cRPA method - Wannier 
function basis - Norm conserving PP's 
(https://journals.jps.jp/doi/abs/10.1143/JPSJ.79.044705).

   When I do it using LRT I get almost doubled? I have checked it w.r.t 
different PP/Exchange-correlation functional here is the results,
   NCPP --> 6.43, USPP --> 6.85, PBESOL --> 6.81, PAW --> 6.83

   > different oxidation states
   I think oxidation state is same in both cases. Fe^2+ for Fe-besed 
superconductors

   > What is important is to use U (for calculations of band structure, 
DOS, phonons, etc.) with the same parameters which which it was 
computed.
   Yes agreed. First of all I like to calculate it for reported values in 
literature then apply it for my calculations.

   Input files are attached (NCPP) for your consideration, that may help 
you to find out my mistakes. I am just a beginner of such calculation 
and don't think without my mistake such large difference can occur 
though U is not an universal parameter.

with many thanks and best regards
Soumyadeep


On 12-09-2019 14:35, Timrov Iurii wrote:
> 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
> -------------------------------------------------------------------
> 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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