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.
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.
Bests,
Lorenzo
Il 20/09/20 10:54, Stefano Baroni ha scritto:
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
—
Stefano Baroni - SISSA, Trieste -
http://stefano.baroni.me, stefanobaroni (Skype)
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]
On 20 Sep 2020, at 10:42, Mohad Abbasnejad
<mohaddeseh.abbasne...@gmail.com
<mailto:mohaddeseh.abbasne...@gmail.com>> wrote:
Hello Dear QE users
I am studying the effect of pressure on the frequencies.
In the experimental article, the pressure on the solid has been
reported to be 7 kbar.
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.
Computing stress (Cartesian axis) and pressure
total stress (Ry/bohr**3) (kbar) P= 6.57
0.00004467 0.00000000 -0.00000000 6.57 0.00 -0.00
0.00000000 0.00004467 -0.00000000 0.00 6.57 -0.00
-0.00000000 -0.00000000 0.00004467 -0.00 -0.00 6.57
MY QUESTION: Which pressure should I report on my paper; 7 kbar or
6.57 kbar?
Any comments would be appreciated.
Best regards
Mohaddeseh
--
---------------------------------------------------------
Mohaddeseh Abbasnejad,
Assistant Professor of Physics,
Faculty of Physics,
Shahid Bahonar University of Kerman,
Kerman, Iran
P.O. Box 76169-133
Tel: +98 34 31322199
Fax: +98 34 33257434
Cellphone: +98 917 731 7514
E-Mail: mohaddeseh.abbasne...@gmail.com
<mailto:mohaddeseh.abbasne...@gmail.com>
Website: academicstaff.uk.ac.ir/moabbasnejad
<http://academicstaff.uk.ac.ir/moabbasnejad>
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