[QE-users] Discrepancy in k-point sampling

Tue Mar 4 14:39:17 CET 2025

Dear Prof. Giovanni,

Thank you for your detailed response; I truly appreciate the time you took
to explain everything thoroughly.

I used vc-relaxed cell parameters with a slight modification in a2 (a(2)_x
= -1/2, a(2)_y = sqrt(3a/2), while the atomic_positions were taken from
VS₂. Then, I performed vc-relax again. Now, the structure is preserving
symmetry.

On Tue, Mar 4, 2025 at 3:15 PM Giovanni Cantele <
giovanni.cantele at spin.cnr.it> wrote:

> The number of k-points is reduced or not reduced according to symmetries.
> So, if for two systems that should be exactly equivalent,
> symmetries are different, that means that either atomic positions are
> different, or lattices are different or both.
> Consider that symmetries are checked according to a threshold. I'm not
> very aware of that, but for sure what the code does is to
> apply symmetry matrices to atomic positions and then compare the obtained
> positions with the initial ones (that should differ by at most lattice
> translations).
> When a comparison is carried out, a threshold is setup, in that quantities
> (in our case atomic positions) are considered numerically the same if they
> differ
> by less than the threshold.
>
> Now, in your case, the first think that comes to the eye is :
> vs2_nscf.out:     12 Sym. Ops. (no inversion) found
> vse2_nscf.out:      2 Sym. Ops. (no inversion) found
>
> So, the two systems definitely have different symmetries, so they are
> different!
> After a closer inspection in the output files (similar conclusion could be
> drawn
> from input files) we see that:
> vs2_nscf.out:
>      crystal axes: (cart. coord. in units of alat)
>                a(1) = (   1.000000   0.000000   0.000000 )
>                a(2) = (  -0.500000   0.866025   0.000000 )
>
> vse2_nscf.out:
>      crystal axes: (cart. coord. in units of alat)
>                a(1) = (   1.000000   0.000000   0.000000 )
>                a(2) = (  -0.500097   0.866068   0.000000 )
>
> As you might see, while in the first case a(2)_x = -1/2, a(2)_y =
> sqrt(3)/2, in the second case there are little deviations
> in the last digits that should be enough to prevent pw.x from finding
> symmetries in the second case.
>
> As a general rule, if I know that a systems is hexagonal, I wouldn't use
> ibrav = 0 if there is a setting (ibrav = 4) that does the job for you!
>
> As you said that you tried ibrav = 4, in that case what could be happened
> is that you forgot to change the atomic positions as well. Indeed,
> those positions are in crystal units, so they were referred to axes that
> were very close to but not exactly (within the threshold) hexagonal ones.
> So,
> he symmetry check, that in the first case was failing because of the
> crystal axes, in the second one is likely to fail because of the atomic
> positions.
>
> Use in the second case ibrav = 4 and exactly the same (in-plane) atomic
> positions as in the first case, and for sure you'll get the same symmetries.
>
> Giovanni
> --
>
> Giovanni Cantele, PhD
> CNR-SPIN
> c/o Dipartimento di Fisica
> Universita' di Napoli "Federico II"
> Complesso Universitario M. S. Angelo - Ed. 6
> Via Cintia, I-80126, Napoli, Italy
> e-mail: giovanni.cantele at spin.cnr.it <giovanni.cantele at spin.cnr.it>
> Phone: +39 081 676910
> Skype contact: giocan74
>
> ResearcherID: http://www.researcherid.com/rid/A-1951-2009
> Web page: https://sites.google.com/view/giovanni-cantele/home
>
>
> Il giorno lun 3 mar 2025 alle ore 17:19 Zimmi Singh <
> singh.60 at kgpian.iitkgp.ac.in> ha scritto:
>
>> Dear QE Community,
>>
>> I am facing an unexpected discrepancy in k-point sampling during
>> calculations for VS₂ and VSe₂, both of which have hexagonal symmetry. I
>> used the same Monkhorst-Pack k-point grid (K_POINTS automatic 18x18x1)
>> for both, yet the number of k-points in the irreducible Brillouin zone
>> (IBZ) differs significantly:
>>
>>    - VS₂: 37 k-points
>>    - VSe₂: 164 k-points
>>
>> To investigate, I also defined the lattice structure of VSe₂ using
>> ibrav=4, but the IBZ k-point count remains 164. I have attached the nscf
>> input and output files for both systems for reference.
>> I would appreciate any insights on this matter, particularly on how to
>> reduce the IBZ k-points for VSe₂, as it is critical for GW calculations. I
>> also appreciate your time and suggestions.
>>
>> --
>> *Best Regards*
>> Zimmi Singh
>>
>> *Research_Scholar *
>>
>>
>> *Department of Metallurgical and Materials Engineering Indian Institute
>> of Technology, Kharagpur Kharagpur, India*
>>
>>
>> _______________________________________________________________________________
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>
>
> _______________________________________________________________________________
> The Quantum ESPRESSO Foundation stands in solidarity with all civilians
> worldwide who are victims of terrorism, military aggression, and
> indiscriminate warfare.
>
> --------------------------------------------------------------------------------
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
> users mailing list users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users

-- 
*Best Regards*
Zimmi Singh

*Research_Scholar *

*Department of Metallurgical and Materials Engineering Indian Institute of
Technology, Kharagpur Kharagpur, India*
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