<div dir="ltr"><div dir="ltr">On Sun, Sep 20, 2020 at 6:39 PM Michal Krompiec <<a href="mailto:michal.krompiec@gmail.com">michal.krompiec@gmail.com</a>> wrote:<br></div><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="auto">ad 1) If I understand correctly, it is because the basis set would change in every step, so the wavefunction from the preceding step couldn’t be used as a starting point for the scf, unless the bands were projected onto the new PW basis.</div></blockquote><div><br></div><div>This is just part of the story, and not the most important one. Working with a fixed basis set ensures a smoother behavior when convergence with respect to the plane-wave cutoff is not very good. In addition, it is much easier to program.</div><div><br></div><div>Paolo<br></div><div><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 dir="auto">Best,</div><div dir="auto">Michal Krompiec</div><div dir="auto">Merck KGaA</div><div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sun, 20 Sep 2020 at 17:25, Robert Molt <<a href="mailto:r.molt.chemical.physics@gmail.com" target="_blank">r.molt.chemical.physics@gmail.com</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 style="overflow-wrap: break-word;">If I may ask a question to clarify a point within this thread…I have always had some confusion on exactly why the the # of basis vectors is not conserved step-to-step. This is an implicit aspect to the question being asked, if I understand properly.<div><br></div><div>1.) Why is the <b>number</b> of plane waves during vc-relax steps constant? Why is not held to a constant energy cutoff, like in an energy calculation? </div><div><br></div><div>My understanding is that ecutrho and ecutwfc “constrain” the number of plane waves during a vc-relax calculation; these two variables completely specify the number of allowed plane waves consistent with the initial unit cell volume. Is this correct?</div><div><br></div><div>My understanding is that if you wanted a constant energy cutoff, you have to vary the number of plane waves, since the volume is changing. Why is this undesirable to do? Is it because comparing different numbers of plane waves results in different energies, such that no fair comparison can be made between calculations with different numbers of basis vectors?</div><div><br></div><div>2.) On this specific question asked by Mohad, I am slightly confused. Is the essence of the problem that the pressure calculation threshold is too low OR is it that you should never use the “properties” other than lattice constants from a vc-relax for the aforementioned reason? In my head, these are seemingly independent issues in how the calculations are working?</div><div><br></div><div>Dr. Robert Molt Jr.</div><div>Indiana University Purdue University</div></div><div style="overflow-wrap: break-word;"><div><br></div><div><div><div><br><blockquote type="cite"><div>On Sep 20, 2020, at 6:03 AM, Lorenzo Monacelli <<a href="mailto:lorenzo.monacelli@roma1.infn.it" target="_blank">lorenzo.monacelli@roma1.infn.it</a>> wrote:</div><br><div><br><br> <br><br> <br><br> <br><br> <div><p>If I interpret correctly the question, you are seeing a P = 6.57<br><br> kbar and not 7 as in the pw.x input press keyword because, when qe<br><br> relax the unit cell, it constrains the basis defined with ecutwfc<br><br> and ecutrho. This basis, as it is formed by plane waves with<br><br> periodic boundaries, depends on the cell size. So the code will<br><br> relax reaching 7 kbar (within the accuracy you chose in the input,<br><br> usually 0.1 kbar), then it will run a new calculation in the final<br><br> cell recomputing the basis with the new cell. So the value of the<br><br> pressure will be slightly different.</p><p>To be consistent, you should use 6.57 kbar in your plots, as it<br><br> is the final pressure with the cutoffs you used for your<br><br> calculation, and does not depend on the starting cell. If you want<br><br> exactly 7 kbar, you can start a new vc-relax using the final<br><br> structure you have at 6.57 kbar as starting point, and it will get<br><br> to a much closer value.</p><p>Bests,</p><p>Lorenzo<br><br><br> </p><br><br> <div>Il 20/09/20 10:54, Stefano Baroni ha<br><br> scritto:<br><br><br> </div><br><br> <blockquote type="cite"><br><br> <br><br> What is difference between the frequencies computed at: i) P=0;<br><br> ii) P=6.57 kbar; iii) 7 kbar? The answer to my question will<br><br> imply that to yours. SB<br><br> <div><br><br><br> <div><br><br> <div style="overflow-wrap: break-word;"><br><br> <div style="font-variant-caps:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><span style="font-size:12px">— <br><br><br> Stefano Baroni - SISSA, Trieste - <a href="http://stefano.baroni.me/" target="_blank">http://stefano.baroni.me</a>, stefanobaroni (Skype) <br><br><br> <br><br><br> If the prediction that an airplane can stay up depends<br><br> on the difference between Riemann and Lebesgue<br><br> integration, I don’t want to fly in it [Richard W.<br><br> Hammings]<br><br><br> </span><br><br><br> <br><br><br> </div><br><br> </div><br><br> </div><br><br> <div><br><br><br> <blockquote type="cite"><br><br> <div>On 20 Sep 2020, at 10:42, Mohad Abbasnejad<br><br> <<a href="mailto:mohaddeseh.abbasnejad@gmail.com" target="_blank">mohaddeseh.abbasnejad@gmail.com</a>><br><br> wrote:</div><br><br> <br><br><br> <div><br><br> <div dir="ltr"><br><br> <div dir="ltr"><br><br> <div>Hello Dear QE users<br><br><br> </div><br><br> <div>I am studying the effect of pressure on<br><br> the frequencies.</div><br><br> <div>In the experimental article, the<br><br> pressure on the solid has been reported to be 7<br><br> kbar.</div><br><br> <div><br><br><br> </div><br><br> <div>When I put this pressure (7 kbar) on my<br><br> structure and let it relax, the final pressure is<br><br> calculated to be 6.57 kbar as the following.<br><br> Therefore it is not exactly 7 kbar.<br><br><br> </div><br><br> <div><br><br><br> </div><br><br> <div>Computing stress (Cartesian axis) and<br><br> pressure<br><br><br> <br><br><br> total stress (Ry/bohr**3) <br><br> (kbar) P= 6.57<br><br><br> 0.00004467 0.00000000 -0.00000000 <br><br> 6.57 0.00 -0.00<br><br><br> 0.00000000 0.00004467 -0.00000000 <br><br> 0.00 6.57 -0.00<br><br><br> -0.00000000 -0.00000000 0.00004467 <br><br> -0.00 -0.00 6.57</div><br><br> <div><br><br><br> </div><br><br> <div><br><br><br> </div><br><br> <div>MY QUESTION: Which pressure should I<br><br> report on my paper; 7 kbar or 6.57 kbar?</div><br><br> <div><br><br><br> </div><br><br> <div><br><br><br> </div><br><br> <div>Any comments would be appreciated.<br><br><br> <br><br><br> </div><br><br> <div>Best regards<br><br><br> </div><br><br> <div>Mohaddeseh<br><br><br> </div><br><br> <br clear="all"><br><br> <br><br><br> -- <br><br><br> <div dir="ltr">---------------------------------------------------------<br><br><br> Mohaddeseh Abbasnejad, <br><br><br> Assistant Professor of Physics,<br><br><br> Faculty of Physics, <br><br><br> Shahid Bahonar University of Kerman,<br><br><br> Kerman, Iran<br><br><br> P.O. Box 76169-133<br><br><br> Tel: +98 34 31322199<br><br><br> Fax: +98 34 33257434<br><br><br> Cellphone: +98 917 731 7514<br><br><br> E-Mail: <a href="mailto:mohaddeseh.abbasnejad@gmail.com" target="_blank">mohaddeseh.abbasnejad@gmail.com</a><br><br><br> Website: <a href="http://academicstaff.uk.ac.ir/moabbasnejad" target="_blank">academicstaff.uk.ac.ir/moabbasnejad</a><br><br> <br><br><br>---------------------------------------------------------<br><br><br> <br><br><br> </div><br><br> </div><br><br> </div><br><br> _______________________________________________<br><br><br> Quantum ESPRESSO is supported by MaX (<a href="http://www.max-centre.eu/quantum-espresso" target="_blank">www.max-centre.eu/quantum-espresso</a>)<br><br><br> users mailing list <a href="mailto:users@lists.quantum-espresso.org" target="_blank">users@lists.quantum-espresso.org</a><br><br><br> <a href="https://lists.quantum-espresso.org/mailman/listinfo/users" target="_blank">https://lists.quantum-espresso.org/mailman/listinfo/users</a></div><br><br> </blockquote><br><br> </div><br><br> <br><br><br> </div><br><br> <br><br><br> <fieldset></fieldset><br><br> <pre>_______________________________________________<br><br>Quantum ESPRESSO is supported by MaX (<a href="http://www.max-centre.eu/quantum-espresso" target="_blank">www.max-centre.eu/quantum-espresso</a>)<br><br>users mailing list <a href="mailto:users@lists.quantum-espresso.org" target="_blank">users@lists.quantum-espresso.org</a><br><br><a href="https://lists.quantum-espresso.org/mailman/listinfo/users" target="_blank">https://lists.quantum-espresso.org/mailman/listinfo/users</a></pre><br><br> </blockquote><br><br> </div><br><br><br><br>_______________________________________________<br>Quantum ESPRESSO is supported by MaX (<a href="http://www.max-centre.eu/quantum-espresso" target="_blank">www.max-centre.eu/quantum-espresso</a>)<br>users mailing list <a href="mailto:users@lists.quantum-espresso.org" target="_blank">users@lists.quantum-espresso.org</a><br><a href="https://lists.quantum-espresso.org/mailman/listinfo/users" target="_blank">https://lists.quantum-espresso.org/mailman/listinfo/users</a></div></blockquote></div><br></div></div></div>_______________________________________________<br><br>Quantum ESPRESSO is supported by MaX (<a href="http://www.max-centre.eu/quantum-espresso" rel="noreferrer" target="_blank">www.max-centre.eu/quantum-espresso</a>)<br><br>users mailing list <a href="mailto:users@lists.quantum-espresso.org" target="_blank">users@lists.quantum-espresso.org</a><br><br><a href="https://lists.quantum-espresso.org/mailman/listinfo/users" rel="noreferrer" target="_blank">https://lists.quantum-espresso.org/mailman/listinfo/users</a></blockquote></div></div>
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