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 < [email protected]> 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: [email protected] <[email protected]> > 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 < > [email protected]> 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* >> >> >> _______________________________________________________________________________ >> 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 [email protected] >> https://lists.quantum-espresso.org/mailman/listinfo/users > > > _______________________________________________________________________________ > 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 [email protected] > 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*
_______________________________________________________________________________ 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 [email protected] https://lists.quantum-espresso.org/mailman/listinfo/users
