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<div dir="ltr" data-setdir="false"><div><p class="ydpa3a80bd6isSelectedEnd">Dear Dr. Timrov,</p><p class="ydpa3a80bd6isSelectedEnd">Thank you very much for your detailed and helpful response.</p><p class="ydpa3a80bd6isSelectedEnd">Your explanation has clarified the role of Hubbard_alpha and the need to avoid treating DFT+U as a black-box method. I will go through the tutorials and papers you recommended, and I will also check the projected density of states to see the contribution of Cu 3d states near the band edges. I also appreciate your comments on the occupations, nbnd, and the use of the QE input generator. </p><p class="ydpa3a80bd6isSelectedEnd">Thank you once again for your time and valuable advice.</p><p>Kind regards,<br>Stephen Mwenda</p></div><br></div><div><br></div>
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On Friday, 15 May 2026 at 18:24:50 GMT+2, Timrov, Iurii <iurii.timrov@psi.ch> wrote:
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Dear Stephen,</div>
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I recommend going through the tutorial that includes YouTube videos for DFT+U:</div>
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<a rel="nofollow noopener noreferrer" shape="rect" target="_blank" href="https://sites.google.com/view/hubbard-koopmans/program">https://sites.google.com/view/hubbard-koopmans/program</a></div>
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Regarding your input files, try to use the QE input generator:</div>
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<a rel="nofollow noopener noreferrer" shape="rect" target="_blank" href="https://qeinputgenerator.materialscloud.io/">https://qeinputgenerator.materialscloud.io/</a></div>
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You can upload your existing input file, and this tool will suggest how to improve it. </div>
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DFT+U and band gaps: If you apply +U to states that form the edges (top of the valence or bottom of the conduction), then the band gap will be changing. Otherwise, if you apply +U to states that are far from the edges, the band gap will not change or change
little (since those deep states that you move with +U will change the hybridization with those states that form the edges). I recommend reading this paper on this topic:</div>
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<a rel="nofollow noopener noreferrer" shape="rect" target="_blank" href="https://www.mdpi.com/2076-3417/11/5/2395">https://www.mdpi.com/2076-3417/11/5/2395</a></div>
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<ol start="1" style="text-align:left;background-color:rgb(255, 255, 255);"><li style="font-family:UI Web -apple-system, BlinkMacSystemFont, Roboto, sans-serif;font-size:15px;color:rgb(36, 36, 36);">
<div>Is it reasonable to apply Hubbard U only to Cu 3d states in Cu2O using this old syntax?</div>
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Check the projected density of states and see where the Cu-3d states are. Are they close to the band edges?</div>
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<div>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?</div>
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Hubbard_alpha is the strength of the perturbation that is used to compute U using linear-response theory based on supercells and finite differences, described here:</div>
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<a rel="nofollow noopener noreferrer" shape="rect" target="_blank" href="https://journals.aps.org/prb/abstract/10.1103/PhysRevB.71.035105">https://journals.aps.org/prb/abstract/10.1103/PhysRevB.71.035105</a></div>
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You need to vary Hubbard_alpha around zero and then compute the response occupations. You need to find the range of Hubbard_alpha values where the response is linear. More is explained in the paper above. </div>
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There is a more efficient way to compute U. Namely, using the HP code, which is based on density functional perturbation theory. It gives the same result as above, but it is computationally faster. Check here if interested:</div>
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<a rel="nofollow noopener noreferrer" shape="rect" target="_blank" href="https://journals.aps.org/prb/abstract/10.1103/PhysRevB.98.085127">https://journals.aps.org/prb/abstract/10.1103/PhysRevB.98.085127</a></div>
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<a rel="nofollow noopener noreferrer" shape="rect" target="_blank" href="https://www.sciencedirect.com/science/article/pii/S0010465522001746">https://www.sciencedirect.com/science/article/pii/S0010465522001746</a></div>
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However, unfortunately, for the system that you want to study (Cu2O) there is a problem with this method. The U will explode if you compute it self-consistently. Read more about this here:</div>
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<a rel="nofollow noopener noreferrer" shape="rect" target="_blank" href="https://pubs.aip.org/aip/jcp/article/140/12/121105/211929/Communication-Comparing-ab-initio-methods-of">https://pubs.aip.org/aip/jcp/article/140/12/121105/211929/Communication-Comparing-ab-initio-methods-of</a></div>
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You can also consider other methods to compute U for such systems (i.e. with closed shells):</div>
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<a rel="nofollow noopener noreferrer" shape="rect" target="_blank" href="https://arxiv.org/abs/2512.16803">https://arxiv.org/abs/2512.16803</a></div>
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<div>Are there any concerns with using occupations = 'fixed' for Cu2O in vc-relax/scf and occupations = 'tetrahedra' for nscf/DOS?</div>
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It is ok since Cu2O is diamagnetic.</div>
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<div>Is nbnd = 40 reasonable for a 6-atom Cu2O cell with 56 electrons for band-structure and DOS analysis?</div>
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You should check how many occupied states there are in this system (it also depends on the pseudopotentials, i.e. whether the semicore states are included or not). Then add some number of empty states. The higher in energy you go for the empty states, the higher
nbnd must be. There is no universal number. It is system-dependent and it depends in which energy interval for PDOS you are interested.</div>
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Are there any syntax issues or methodological concerns that I should correct before finalizing the calculations?</li></ol>
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Regarding the syntax, use QE input generator. Regarding the methodological concerns, you should not use DFT+U as a black box. There are many intricacies that one should be aware of before using this method. It looks simple, but at the same time it is very tricky. </div>
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HTH</div>
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Greetings,</div>
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Iurii</div>
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<div style="font-family:Cambria, Georgia, serif;font-size:12pt;color:rgb(102, 102, 102);" class="yiv0787238831elementToProof">
Dr. Iurii TIMROV</div>
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Tenure-track scientist</div>
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Laboratory for Materials Simulations (LMS)</div>
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Paul Scherrer Institute (PSI)</div>
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CH-5232 Villigen, Switzerland</div>
<div style="font-size:12pt;" class="yiv0787238831elementToProof"><span style="font-family:Cambria, Georgia, serif;color:rgb(102, 102, 102);">Profile:
</span><span style="font-family:Aptos, Calibri, Helvetica, sans-serif;color:rgb(0, 0, 0);"><a rel="nofollow noopener noreferrer" shape="rect" target="_blank" href="https://www.psi.ch/en/lms/people/iurii-timrov">www.psi.ch/en/lms/people/iurii-timrov</a></span></div>
<div style="font-size:12pt;" class="yiv0787238831elementToProof"><span style="font-family:Cambria, Georgia, serif;color:rgb(102, 102, 102);">Group website:
</span><span style="font-family:Aptos, Calibri, Helvetica, sans-serif;color:rgb(0, 0, 0);"><a rel="nofollow noopener noreferrer" shape="rect" target="_blank" href="https://www.timrovresearch.com/">www.timrovresearch.com/</a></span></div>
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<div id="yiv0787238831yqt57247" class="yiv0787238831yqt8990787303"><div dir="ltr" id="yiv0787238831divRplyFwdMsg"><font face="Calibri, sans-serif" style="font-size:11pt;" color="#000000"><b>From:</b> users <users-bounces@lists.quantum-espresso.org> on behalf of Stephen Mwenda via users <users@lists.quantum-espresso.org><br clear="none">
<b>Sent:</b> Friday, May 15, 2026 17:46<br clear="none">
<b>To:</b> users@lists.quantum-espresso.org <users@lists.quantum-espresso.org><br clear="none">
<b>Subject:</b> [QE-users] Advice on Cu2O DFT+U input setup in Quantum ESPRESSO</font>
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<p class="yiv0787238831x_isSelectedEnd" style="margin-top: 0px; margin-bottom: 0px;"><span>Dear Quantum ESPRESSO users,</span></p>
<p class="yiv0787238831x_isSelectedEnd" style="margin-top: 0px; margin-bottom: 0px;"><span>I hope you are well.</span></p>
<p class="yiv0787238831x_isSelectedEnd" style="margin-top: 0px; margin-bottom: 0px;"><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="yiv0787238831x_isSelectedEnd" style="margin-top: 0px; margin-bottom: 0px;"><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="yiv0787238831x_isSelectedEnd" style="margin-top: 0px; margin-bottom: 0px;"><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="yiv0787238831x_isSelectedEnd" style="margin-top: 0px; margin-bottom: 0px;"><span>My calculation setup is as follows:</span></p>
<ul><li><span>Material: Cu2O, cubic cuprite structure</span></li><li><span>Exchange-correlation functional: PBE</span></li><li><span>Pseudopotentials:</span>
<ul><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="yiv0787238831x_isSelectedEnd" style="margin-top: 0px; margin-bottom: 0px;"><span>For U = 1 eV, the relevant part of my &SYSTEM block is:</span></p>
<p class="yiv0787238831x_isSelectedEnd" style="margin-top: 0px; margin-bottom: 0px;"><span>&SYSTEM</span><br clear="none">
<span>ibrav = 1</span><br clear="none">
<span>a = 4.25220</span><br clear="none">
<span>nat = 6</span><br clear="none">
<span>ntyp = 2</span><br clear="none">
<span>ecutwfc = 60</span><br clear="none">
<span>ecutrho = 480</span><br clear="none">
<span>occupations = 'fixed'</span><br clear="none">
<span>lda_plus_u = .true.</span><br clear="none">
<span>Hubbard_U(1) = 1.0</span><br clear="none">
<span>Hubbard_alpha(1) = -0.08</span><br clear="none">
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<p class="yiv0787238831x_isSelectedEnd" style="margin-top: 0px; margin-bottom: 0px;"><span>My ATOMIC_SPECIES block is:</span></p>
<p class="yiv0787238831x_isSelectedEnd" style="margin-top: 0px; margin-bottom: 0px;"><span>ATOMIC_SPECIES</span><br clear="none">
<span>Cu 63.54600 Cu.pbe-dn-rrkjus_psl.1.0.0.UPF</span><br clear="none">
<span>O 15.99940 O.pbe-n-rrkjus_psl.1.0.0.UPF</span></p>
<p class="yiv0787238831x_isSelectedEnd" style="margin-top: 0px; margin-bottom: 0px;"><span>Therefore, Hubbard_U(1) and Hubbard_alpha(1) are intended to apply to Cu.</span></p>
<p class="yiv0787238831x_isSelectedEnd" style="margin-top: 0px; margin-bottom: 0px;"><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="yiv0787238831x_isSelectedEnd" style="margin-top: 0px; margin-bottom: 0px;"><span>My specific questions are:</span></p>
<ol 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="yiv0787238831x_isSelectedEnd" style="margin-top: 0px; margin-bottom: 0px;"><span>I would be grateful for any advice or corrections from experienced users.</span></p>
<p style="margin-top: 0px; margin-bottom: 0px;"><span>Kind regards,</span><br clear="none">
<span>Stephen</span></p>
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