<div dir="ltr"><div>Iurii is perfectly right: U is not a portable parameter. This being said, it seems to me strange that two calculations performed with different approaches but with the same value of U may yield opposite results. It seems to me more likely that one of the two calculations is not correctly set up.</div><div><br></div><div>Paolo<br></div><br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, May 6, 2020 at 10:59 AM Timrov Iurii <<a href="mailto:iurii.timrov@epfl.ch">iurii.timrov@epfl.ch</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
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<p>Dear Shivesh,</p>
<p><br>
</p>
<p><span>> While performing PBE+U calculations on a 2-D material, I wanted to try using NCPPs and then PAW PPs. For the former, I obviously tried QE and for the latter, VASP.
<br>
</span></p>
<p><span><br>
</span></p>
<p><span>In Quantum ESPRESSO, DFT+U can be used with any type of pseudopotentials (NC PPs, US PPs, PAW PPs). So your motivation to switch to VASP because of PAW PPs looks strange to me.<br>
</span></p>
<p><span><br>
</span></p>
<p><span>However, it is important to note that DFT+U for PAW PPs in QE and VASP is not implemented in the same way. There are some technical differences (in QE the projections are done on atomic/ortho-atomic orbitals while in VASP on \beta projector functions).
This aspect will be discussed briefly in the paper which we are working on (Timrov, Marzari, Cococcioni).<br>
</span></p>
<p><span><br>
</span></p>
<p><span>> The electronic bandstructures look very similar (the parameters are completely the same, except for plane wave cutoffs, as you might imagine) but the band gap is different.
<br>
</span></p>
<p><span><br>
</span></p>
<p><span>Which parameters are completely the same? Do you refer to a k points sampling, smearing, lattice parameter? Or Hubbard U?
<br>
</span></p>
<p><span><br>
</span></p>
<p><span>> With NCPPs, the Hubbard 'U' seems to suppress the band gap (compared to U=0) whereas for VASP calculation, it seems to increase the band gap (compared to U=0). Is there a qualitative explanation for this?</span><br>
</p>
<p><br>
</p>
<p>Which values of U do you use in the two codes? How did you choose them?</p>
<p><br>
</p>
<p>Please note that <b>the Hubbard U parameter is not a universal parameter</b>. If you compute it from first principles (e.g. using linear response theory, cRPA, or other approach), its value depends on many factors (type of the Hubbard manifold, pseudopotentials,
functional, oxidation state, etc.). So if you use the same value of U in two different codes (i.e. different implementations of DFT+U), with different PPs, with different Hubbard manifolds, etc., then the effect of the Hubbard correction with same U will be
different. <br>
</p>
<p><br>
</p>
<p>The correct procedure is the following: to compute Hubbard parameters from first-principles for a given set of computational parameters (Hubbard manifold, PPs, functional, etc.), and
<b>use it consistently with exactly the same set of parameters</b>. In QE, Hubbard parameters can be computed using the HP code. Hubbard U is not global, and hence it is not portable (i.e. you cannot compute it in one code with one set of parameters, and then
use it in another code with another set of parameters; even in the same code you cannot compute with one set of parameters and then use it in the same code with another set of parameters).</p>
<p><br>
</p>
<p>Greetings,</p>
<p>Iurii<br>
</p>
<p><br>
</p>
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<div id="gmail-m_-3364848059559674233divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" face="Calibri, sans-serif" color="#000000"><b>From:</b> users <<a href="mailto:users-bounces@lists.quantum-espresso.org" target="_blank">users-bounces@lists.quantum-espresso.org</a>> on behalf of Shivesh Sivakumar <<a href="mailto:shiveshsivakumar@gmail.com" target="_blank">shiveshsivakumar@gmail.com</a>><br>
<b>Sent:</b> Wednesday, May 6, 2020 12:35:30 AM<br>
<b>To:</b> Quantum ESPRESSO users Forum<br>
<b>Subject:</b> [QE-users] Different implementations of the Hubbard 'U' between QE and VASP</font>
<div> </div>
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<div>
<div dir="ltr">Hello all,<br>
<br>
While performing PBE+U calculations on a 2-D material, I wanted to try using NCPPs and then PAW PPs. For the former, I obviously tried QE and for the latter, VASP. The electronic bandstructures look very similar (the parameters are completely the same, except
for plane wave cutoffs, as you might imagine) but the band gap is different.
<div>With NCPPs, the Hubbard 'U' seems to suppress the band gap (compared to U=0) whereas for VASP calculation, it seems to increase the band gap (compared to U=0). Is there a qualitative explanation for this?<br>
<br>
I am very sorry if this question is not totally appropriate for the forum but I just want to know if conceptually, there are some differences in implementation that I am missing out.<br>
<br>
Best,</div>
<div>Shivesh Sivakumar<br>
University of Washington<br>
Seattle, WA-98105</div>
</div>
</div>
</div>
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