<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">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 class=""><br class=""></div><div class="">1.) Why is the <b class="">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 class=""><br class=""></div><div class="">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 class=""><br class=""></div><div class="">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 class=""><br class=""></div><div class="">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 class=""><br class=""></div><div class="">Dr. Robert Molt Jr.</div><div class="">Indiana University Purdue University</div><div class=""><br class=""></div><div class=""><div class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Sep 20, 2020, at 6:03 AM, Lorenzo Monacelli <<a href="mailto:lorenzo.monacelli@roma1.infn.it" class="">lorenzo.monacelli@roma1.infn.it</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" class="">
<div class=""><p class="">If I interpret correctly the question, you are seeing a P = 6.57
kbar and not 7 as in the pw.x input press keyword because, when qe
relax the unit cell, it constrains the basis defined with ecutwfc
and ecutrho. This basis, as it is formed by plane waves with
periodic boundaries, depends on the cell size. So the code will
relax reaching 7 kbar (within the accuracy you chose in the input,
usually 0.1 kbar), then it will run a new calculation in the final
cell recomputing the basis with the new cell. So the value of the
pressure will be slightly different.</p><p class="">To be consistent, you should use 6.57 kbar in your plots, as it
is the final pressure with the cutoffs you used for your
calculation, and does not depend on the starting cell. If you want
exactly 7 kbar, you can start a new vc-relax using the final
structure you have at 6.57 kbar as starting point, and it will get
to a much closer value.</p><p class="">Bests,</p><p class="">Lorenzo<br class="">
</p>
<div class="moz-cite-prefix">Il 20/09/20 10:54, Stefano Baroni ha
scritto:<br class="">
</div>
<blockquote type="cite" cite="mid:D9FF9ACF-8D46-4D48-85E3-6F03FC8FE8E0@sissa.it" class="">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" class="">
What is difference between the frequencies computed at: i) P=0;
ii) P=6.57 kbar; iii) 7 kbar? The answer to my question will
imply that to yours. SB
<div class=""><br class="">
<div class="">
<div style="word-wrap: break-word; -webkit-nbsp-mode: space;
-webkit-line-break: after-white-space;" class="">
<div style="font-variant-caps: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-size: 12px;" class="">— <br class="">
Stefano Baroni - SISSA, Trieste - <a href="http://stefano.baroni.me/" class="" moz-do-not-send="true">http://stefano.baroni.me</a>, stefanobaroni (Skype) <br class="">
<br class="">
If the prediction that an airplane can stay up depends
on the difference between Riemann and Lebesgue
integration, I don’t want to fly in it [Richard W.
Hammings]<br class="">
</span><br class="">
<br class="">
</div>
</div>
</div>
<div class=""><br class="">
<blockquote type="cite" class="">
<div class="">On 20 Sep 2020, at 10:42, Mohad Abbasnejad
<<a href="mailto:mohaddeseh.abbasnejad@gmail.com" class="" moz-do-not-send="true">mohaddeseh.abbasnejad@gmail.com</a>>
wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<div dir="ltr" class="">
<div dir="ltr" class="">
<div class="">Hello Dear QE users<br class="">
</div>
<div class="">I am studying the effect of pressure on
the frequencies.</div>
<div class="">In the experimental article, the
pressure on the solid has been reported to be 7
kbar.</div>
<div class=""><br class="">
</div>
<div class="">When I put this pressure (7 kbar) on my
structure and let it relax, the final pressure is
calculated to be 6.57 kbar as the following.
Therefore it is not exactly 7 kbar.<br class="">
</div>
<div class=""><br class="">
</div>
<div class="">Computing stress (Cartesian axis) and
pressure<br class="">
<br class="">
total stress (Ry/bohr**3)
(kbar) P= 6.57<br class="">
0.00004467 0.00000000 -0.00000000
6.57 0.00 -0.00<br class="">
0.00000000 0.00004467 -0.00000000
0.00 6.57 -0.00<br class="">
-0.00000000 -0.00000000 0.00004467
-0.00 -0.00 6.57</div>
<div class=""><br class="">
</div>
<div class=""><br class="">
</div>
<div class="">MY QUESTION: Which pressure should I
report on my paper; 7 kbar or 6.57 kbar?</div>
<div class=""><br class="">
</div>
<div class=""><br class="">
</div>
<div class="">Any comments would be appreciated.<br class="">
<br class="">
</div>
<div class="">Best regards<br class="">
</div>
<div class="">Mohaddeseh<br class="">
</div>
<br class="" clear="all">
<br class="">
-- <br class="">
<div dir="ltr" class="gmail_signature">---------------------------------------------------------<br class="">
Mohaddeseh Abbasnejad, <br class="">
Assistant Professor of Physics,<br class="">
Faculty of Physics, <br class="">
Shahid Bahonar University of Kerman,<br class="">
Kerman, Iran<br class="">
P.O. Box 76169-133<br class="">
Tel: +98 34 31322199<br class="">
Fax: +98 34 33257434<br class="">
Cellphone: +98 917 731 7514<br class="">
E-Mail: <a href="mailto:mohaddeseh.abbasnejad@gmail.com" target="_blank" class="" moz-do-not-send="true">mohaddeseh.abbasnejad@gmail.com</a><br class="">
Website: <a href="http://academicstaff.uk.ac.ir/moabbasnejad" target="_blank" class="" moz-do-not-send="true">academicstaff.uk.ac.ir/moabbasnejad</a>
<br class="">
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Quantum ESPRESSO is supported by MaX (<a href="http://www.max-centre.eu/quantum-espresso" class="" moz-do-not-send="true">www.max-centre.eu/quantum-espresso</a>)<br class="">
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