<div dir="ltr">Hi,<div><br></div><div>users writing on this forum are usually requested to sign their post with name and affiliation.</div><div><br></div><div>This being said, what you find can be perfectly logic.</div><div><br></div><div>You have to distinguish between ecutrho and ecutwfc.</div><div><br></div><div>The total energy is expected to have a "variational" behavior with respect to ecutwfc, because ecutwfc controls the size of the plane wave basis set: the larger ecutwfc, the larger the basis set, the smaller the total energy. At which values of ecutwfc should one stop? This can be controlled by looking at the convergence of calculated properties (e.g. forces of energy differences or phonon frequencies, and so on, depending on what you're interested in).</div><div><br></div><div>A different logic must be applied to ecutrho. Indeed, ecutrho controls the representation of the part of the charge density that is not expected to change significantly during the scf cycle and that is added to the total charge density at the end of each scf cycle. For norm-conserving pseudopotential ecuttho=4*ecutwfc exactly reproduces the charge density, whereas for pseudopotentials that are not norm conserving you need more plane waves to represent the charge density localized on the atomic cores. As such, you fix ecutrho = 6, 8, 12, .... times ecutwfc (see also here: <a href="https://www.quantum-espresso.org/Doc/INPUT_PW.html#idm301">https://www.quantum-espresso.org/Doc/INPUT_PW.html#idm301</a>). So, saying that you find a minimum total energy with respect to ecutrho is strictly speaking "wrong", because this parameter does not control the size of the basis set but only the accuracy with which you represent the charge density. So, the purpose is that of finding a value of ecutrho at which you reach a satisfactory convergence, not the minimum energy. The total energy, as a function of ecutrho, can either increase or decrease, because it is a matter of numerical accuracy (good versus band representation of the charge density), not of basis set size.</div><div><br></div><div>Giovanni</div><div><br></div><div><div><div><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr"><span style="color:rgb(0,0,0)">-- <br><br>Giovanni Cantele, PhD<br>CNR-SPIN<br>c/o Dipartimento di Fisica<br>Universita' di Napoli "Federico II"<br>Complesso Universitario M. S. Angelo - Ed. 6<br>Via Cintia, I-80126, Napoli, Italy<br><a href="mailto:giovanni.cantele@spin.cnr.it" style="color:rgb(17,85,204)" target="_blank">e-mail: giovanni.cantele@spin.cnr.it</a><br>Phone: +39 081 676910<br>Skype contact: giocan74<br><br>ResearcherID: <a href="http://www.researcherid.com/rid/A-1951-2009" style="color:rgb(17,85,204)" target="_blank">http://www.researcherid.com/rid/A-1951-2009</a><br>Web page: </span><a href="https://sites.google.com/view/giovanni-cantele/home" style="color:rgb(17,85,204)" target="_blank">https://sites.google.com/view/giovanni-cantele/home</a><br></div></div></div><br></div></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">Il giorno mar 21 feb 2023 alle ore 10:56 NM SALMA <<a href="mailto:rose-marron2008@hotmail.fr">rose-marron2008@hotmail.fr</a>> ha scritto:<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 class="msg5055209548081122881">
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Hello,</div>
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I was doing the convergence testing of CuI material.</div>
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For a fixed Ecutwfc value, when I increase the Ecutrho the total energy of the system increase. I was expecting the inverse.</div>
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For example for Ecutwfc=90 Ry and Ecutrho= 180 Ry the total energy= -2368.11704109 Ry<br>
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and for Ecutwfc=90 Ry and Ecutrho=450 Ry the total energy= -2368.11613851 Ry.</div>
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Is that logic? I thought that an increase in the basis-set minimize the system energy.</div>
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I also remarked in the pseudopotential file that :</div>
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For Cu : - Suggested minimum cutoff for wavefunctions: 45. Ry<br>
- Suggested minimum cutoff for charge density: 236. Ry</div>
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For I : - Suggested minimum cutoff for wavefunctions: 24. Ry<br>
- Suggested minimum cutoff for charge density: 109. Ry</div>
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So how it can be that the suggested minimum cutoff for charge density of Cu is 236 Ry, while I found that the system have a minimum total energy for Ecutrho=180 Ry<br>
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