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

[Robert Molt]

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 <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 <mailto: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 <mailto:mohaddeseh.abbasnejad at gmail.com>
>>> Website:  academicstaff.uk.ac.ir/moabbasnejad <http://academicstaff.uk.ac.ir/moabbasnejad> 
>>> ---------------------------------------------------------
>>> 
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