[QE-users] Which pressure should I report on my work???

Sun Sep 20 18:38:50 CEST 2020

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.
Best,
Michal Krompiec
Merck KGaA

On Sun, 20 Sep 2020 at 17:25, Robert Molt <r.molt.chemical.physics at gmail.com>
wrote:

> 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.
>
> 1.) Why is the *number* of plane waves during vc-relax steps constant?
> Why is not held to a constant energy cutoff, like in an energy calculation?
>
> 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?
>
> 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?
>
> 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?
>
> Dr. Robert Molt Jr.
> Indiana University Purdue University
>
>
> On Sep 20, 2020, at 6:03 AM, Lorenzo Monacelli <
> lorenzo.monacelli at roma1.infn.it> wrote:
>
>
>
>
>
>
>
>
>
> 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]
>
>
>
>
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>
> On 20 Sep 2020, at 10:42, Mohad Abbasnejad
>
> <mohaddeseh.abbasnejad at 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.abbasnejad at gmail.com
>
>
> Website:  academicstaff.uk.ac.ir/moabbasnejad
>
>
>
>
> ---------------------------------------------------------
>
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