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

Thu Sep 12 14:03:45 CEST 2019

Dear Soumyadeep,

Unfortunately I cannot open the input file "LiFeAs-0GPa.scf". Could you please share it via Google Drive or Dropbox? Is you system magnetic, metallic?

The value of U is very sensitive to the type of localized functions which are used, so we have Wannier in cRPA vs atomic/ortho-atomic in LRT. That may make a big difference, I think.

Another aspect, comparison of methods cRPA vs constrained DFT (LRT). There were some attempts to obtain same U with these two methods, but nobody succeeded to so far, to the best of my knowledge. In particular, here is an important paper concerning this issue: Phys. Rev. B 74, 125106 (2006). In Fig. 2 of this paper you can see that cRPA gives the value of U which is very sensitive to the fact which electrons are allowed to screen the d-d interactions. So in your case, I would try to understand how U was computed with cRPA (if the authors provide enough details in the publication which you cited), i.e. which electrons were allowed to screen. In the LRT approach instead, there is no this kind of choice which electrons are allowed to screen: when one perturbs e.g. d electronic states of Fe then all other electrons in the system are allowed to screen (this is done via the self-consistent solution of modified Kohn-Sham equations). To conclude, comparing cRPA with LRT is not an easy task (at least I do not know any publication which clearly makes such a comparison, apart from the one which I mentioned above).

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: Soumyadeep <soumyadeep at rrcat.gov.in>
Sent: Thursday, September 12, 2019 12:56:03 PM
To: Quantum ESPRESSO users Forum
Cc: Timrov Iurii
Subject: Re: [QE-users] hp.x code overestimates Hubbard U value

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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