<!DOCTYPE html>
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
</head>
<body>
<p class="isSelectedEnd"><span>Dear Quantum ESPRESSO users,</span></p>
<p class="isSelectedEnd"><span>I hope you are well.</span></p>
<p class="isSelectedEnd"><span>I am carrying out DFT calculations on
cuprous oxide, Cu2O, using Quantum ESPRESSO. My aim is to study
how the electronic band gap changes with Hubbard U applied to
the Cu 3d states.</span></p>
<p class="isSelectedEnd"><span>I would like to kindly ask for advice
on whether my input setup is reasonable, especially regarding
the old-style DFT+U syntax, pseudopotentials, and use of the
Hubbard alpha parameter.</span></p>
<p class="isSelectedEnd"><span>I have attached one representative
input file for the vc-relax calculation at U = 1 eV, since this
file includes the Hubbard U setup. The same workflow is then
repeated for U = 2 to 6 eV by changing the value of
Hubbard_U(1). For U = 0, no Hubbard correction is applied.</span></p>
<p class="isSelectedEnd"><span>My calculation setup is as follows:</span></p>
<ul data-spread="false">
<li><span>Material: Cu2O, cubic cuprite structure</span></li>
<li><span>Exchange-correlation functional: PBE</span></li>
<li><span>Pseudopotentials:</span>
<ul data-spread="false">
<li><span>Cu.pbe-dn-rrkjus_psl.1.0.0.UPF</span></li>
<li><span>O.pbe-n-rrkjus_psl.1.0.0.UPF</span></li>
</ul>
</li>
<li><span>ecutwfc = 60 Ry</span></li>
<li><span>ecutrho = 480 Ry</span></li>
<li><span>k-points for vc-relax and scf: 6 6 6 0 0 0</span></li>
<li><span>k-points for nscf/DOS: 12 12 12 0 0 0</span></li>
<li><span>Number of atoms: 6</span></li>
<li><span>Number of bands in nscf: 40</span></li>
</ul>
<p class="isSelectedEnd"><span>For U = 1 eV, the relevant part of my
&SYSTEM block is:</span></p>
<p class="isSelectedEnd"><span>&SYSTEM</span><br>
<span>ibrav = 1</span><br>
<span>a = 4.25220</span><br>
<span>nat = 6</span><br>
<span>ntyp = 2</span><br>
<span>ecutwfc = 60</span><br>
<span>ecutrho = 480</span><br>
<span>occupations = 'fixed'</span><br>
<span>lda_plus_u = .true.</span><br>
<span>Hubbard_U(1) = 1.0</span><br>
<span>Hubbard_alpha(1) = -0.08</span><br>
<span>/</span></p>
<p class="isSelectedEnd"><span>My ATOMIC_SPECIES block is:</span></p>
<p class="isSelectedEnd"><span>ATOMIC_SPECIES</span><br>
<span>Cu 63.54600 Cu.pbe-dn-rrkjus_psl.1.0.0.UPF</span><br>
<span>O 15.99940 O.pbe-n-rrkjus_psl.1.0.0.UPF</span></p>
<p class="isSelectedEnd"><span>Therefore, Hubbard_U(1) and
Hubbard_alpha(1) are intended to apply to Cu.</span></p>
<p class="isSelectedEnd"><span>The relaxed structure remains cubic,
and the calculated direct band gap at Γ increases from about
0.46 eV at U = 0 to about 0.64 eV at U = 6 eV. I understand that
PBE/PBE+U may still underestimate the experimental Cu2O band
gap, but I would like to confirm whether the input syntax and
general setup are technically correct.</span></p>
<p class="isSelectedEnd"><span>My specific questions are:</span></p>
<ol data-spread="false" start="1">
<li><span>Is it reasonable to apply Hubbard U only to Cu 3d states
in Cu2O using this old syntax?</span></li>
<li><span>Is the use of Hubbard_alpha(1) = -0.08 appropriate in a
production DFT+U calculation, or should it only be used for
linear-response/perturbative calculations?</span></li>
<li><span>Are there any concerns with using occupations = 'fixed'
for Cu2O in vc-relax/scf and occupations = 'tetrahedra' for
nscf/DOS?</span></li>
<li><span>Is nbnd = 40 reasonable for a 6-atom Cu2O cell with 56
electrons for band-structure and DOS analysis?</span></li>
<li><span>Are there any syntax issues or methodological concerns
that I should correct before finalizing the calculations?</span></li>
</ol>
<p class="isSelectedEnd"><span>I would be grateful for any advice or
corrections from experienced users.</span></p>
<p><span>Kind regards,</span><br>
<span>Stephen</span></p>
<br>
</body>
</html>