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

[Lorenzo Monacelli]

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 
>> <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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