[QE-users] Query regarding ecutfock

[Lorenzo Bastonero]

Dear Arini,

As a final remark, this is what experience tells us, but remember that proper testing should be directly performed on the desired property with a desired accuracy.
This means that a better practice could be e.g. to test  ``ecutfock`` on phonon frequencies at Gamma (i.e. only displacements in UNITCELL). This already gives a better idea of convergence, while still not going for expensive supercells.

If you are interested in more details, I’d suggest you the following paper, which I found particularly instructive: https://www.nature.com/articles/s41524-018-0127-2 <https://www.nature.com/articles/s41524-018-0127-2>

Bests,
Lorenzo

********************************
Lorenzo Bastonero

PhD Student
U Bremen Excellence Chair,
Bremen Center for Computational Materials Science,
and MAPEX Center for Materials and Processes

University of Bremen
Faculty of Production Engineering
TAB-Building, Room 3.32
Am Fallturm 1
28359 Bremen, DE

http://www.hmi.uni-bremen.de/

> Il giorno 12 apr 2023, alle ore 14:08, Arini Kar <16i170008 at iitb.ac.in> ha scritto:
> 
> Dear Lorenzo,
> 
> Your suggestion was helpful. The forces on the pristine unit cell did not change after reducing ecutfock to 2*ecutwfc. Thanks a lot.
> 
> 
> 
> Regards
> Arini
> 
> 
> 
> On 2023-04-12 13:26, Lorenzo Bastonero wrote:
> 
>> Dear Arini,
>>  
>> You should do some test first in the pristine unit cell structure before going forward to the supercell calculations. 
>>  
>> In principle, you could lower down even more that cutoff (and the qpoint for the fock operator as well). Usually a good threshold is ~10^-5 Ry/bohr on every atomic force component.
>>  
>> Note: this threshold is different from the one of the relax. So you should check that when changing your variables (ecutfock in this case) all your forces don't change within this threshold.
>>  
>> HTH,
>> Lorenzo 
>> 
>> Inviato da iPhone
>> 
>>> Il giorno 11 apr 2023, alle ore 20:45, Arini Kar via users <users at lists.quantum-espresso.org> ha scritto:
>>> 
>>> Dear users,
>>> 
>>> I am trying to run phonon calculations using phonopy. I am using norm conserving pseudopotential. Can I use ecutfock = 2*ecutwfc instead of the default value in the scf calculations? Would it affect the accuracy of force calculations?
>>> 
>>> Thanking you.
>>> 
>>> Regards
>>> 
>>> Arini Kar
>>> 
>>> PhD student 
>>> 
>>> Indian Institute of Technology Bombay, India
>>> 
>>> 
>>> 
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