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

[Paolo Giannozzi]

On Sun, Sep 20, 2020 at 6:39 PM Michal Krompiec <michal.krompiec at gmail.com>
wrote:

> 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.
>

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.

Paolo

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]
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> 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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>>
>>
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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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