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<p>Dear Soumyadeep,</p>
<p><br>
</p>
<p>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:</p>
<p>- Manifold of localized functions (atomic, ortho-atomic, Wannier, etc.);</p>
<p>- Pseudopotentials;</p>
<p>- Oxidation state;</p>
<p>- Exchange-correlation functional (LDA, PBE, PBEsol, etc.);</p>
<p>- Environment (e.g. U for Mn-3d in MnO and BiMnO3 is not exactly the same).</p>
<p><br>
</p>
<p>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.</p>
<p><br>
</p>
<p>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).</p>
<p><br>
</p>
<p><font size="2"><span style="font-size:10pt"><span style="font-size:12pt">> ...</span><span style="font-size:12pt">"U" lies between 3.0-3.5 eV...</span></span></font><br>
</p>
<p><br>
</p>
<p>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.</p>
<p><br>
</p>
<p>I recommend to have a look at the hands-on about calculation of U using Quantum ESPRESSO (hp.x code):</p>
<p><a href="https://agenda.ethernet.edu.et/event/33/" class="x_OWAAutoLink" id="LPlnk660304">https://agenda.ethernet.edu.et/event/33/</a></p>
<p><br>
</p>
<p>Hope this helps! If something is not precise in my explanations, it would great if other experts correct me.</p>
<p><br>
</p>
<p>Best regards,</p>
<p>Iurii<br>
</p>
<p><br>
</p>
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<div id="x_divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> users <users-bounces@lists.quantum-espresso.org> on behalf of Soumyadeep <soumyadeep@rrcat.gov.in><br>
<b>Sent:</b> Wednesday, September 11, 2019 4:35:37 PM<br>
<b>To:</b> Users<br>
<b>Subject:</b> [QE-users] hp.x code overestimates Hubbard U value</font>
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<font size="2"><span style="font-size:10pt;">
<div class="PlainText">Dear QE users,<br>
<br>
I am getting a overestimated value of Hubbard parameters using hp.x <br>
executable of Quantum Espresso. My material is iron based <br>
superconductors (LiFeAs) for which "U" lies between 3.0-3.5 eV, but <br>
using this code I get it around 6.4-6.5 eV.<br>
<br>
Can anyone find out possible mistakes?<br>
<br>
with many thanks and best regards<br>
Soumyadeep<br>
-------------------------------------------------------------------<br>
Soumyadeep Ghosh,<br>
Senior Research Fellow,<br>
Homi Bhabha National Institute (HBNI),<br>
Raja Ramanna Centre for Advanced Technology, Indore, India-452013<br>
Mob: (+91)9424664553<br>
User Lab: 0731244-2580<br>
Email: soumyadeepghosh35@gmail.com, soumyadeep@rrcat.gov.in<br>
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