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<p>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>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>Bests,</p>
<p>Lorenzo<br>
</p>
<div class="moz-cite-prefix">Il 20/09/20 10:54, Stefano Baroni ha
scritto:<br>
</div>
<blockquote type="cite"
cite="mid:D9FF9ACF-8D46-4D48-85E3-6F03FC8FE8E0@sissa.it">
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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="">
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letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;"><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>
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<div><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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