[QE-users] Symmetry Not Recognized in SCF Calculation for a 2D Carbon Allotrope (ibrav=6)

[Giovanni Cantele]

Hi,

it is exactly as you expect. Indeed, QE is able to recognize the symmetry
based on the structure.

However, symmetry check is based on atomic coordinate comparisons, in that
a position and its symmetry-related
counterpart are compared and supposed to be equivalent using a given
threshold.

That means that if, for example in bulk silicon, you use the atomic
positions (in crystal units)
Si    0.0.   0.0.   0.00
Si    0.25  0.25  0.25
symmetries will be correctly found but, if for some reason, you use for
example
Si    0.0.   0.0.   0.00
Si    0.25  0.25  0.2501
some (or all) of them will be lost.

Returning to your systems, I understand that it is the result of a vc-relax
optimization and from your atomic positions
I find some that look very similar but are not exactly the same.
For example: should 3.9245618110 and 3.9253683520 be the same in the ideal
symmetric structure?
Similarly: 4.2961516297 and 4.2953414224 and other examples (including all
z coordinates).

So if you expect that those coordinates should be symmetry related, your
missing symmetries are only due to
some "numerical noise" affecting your atomic positions. In this case you
can try to retrieve symmetries by adjusting coordinates
according to what you expect.

If instead, this is not the case, you should explore the possibility that
the structure has not that symmetry because  one or
more atoms have wrong positions.

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 ven 28 mar 2025 alle ore 10:29 Zimmi Singh <
singh.60 at kgpian.iitkgp.ac.in> ha scritto:

> Dear QE community,
>
> I’m encountering an issue with symmetry detection in my SCF and NSCF
> calculations for a 2D carbon allotrope using Quantum ESPRESSO (v.6.8). I’ve
> performed a vc-relax calculation to obtain relaxed lattice parameters,
> which I then used in my SCF input. However, the SCF output indicates "No
> symmetry found," despite setting ibrav=6 (tetragonal symmetry). I
> expected QE to recognize the symmetry based on the structure.
>
> The point group is identified as C_1 (1) with only the identity
> operation, which suggests QE isn’t detecting the expected tetragonal
> symmetry (P4mm space group). I have attached the link
> <https://drive.google.com/drive/folders/1m2z9bF6pdf1q9_59b77i4FjSo4wua4-j?usp=sharing>
> for SCF input and output files for your reference. Any insights or
> suggestions on ensuring symmetry is recognized would be greatly appreciated.
> link:
> https://drive.google.com/drive/folders/1m2z9bF6pdf1q9_59b77i4FjSo4wua4-j?usp=sharing
>
> --
> *Best Regards*
> Zimmi Singh
>
> *Research_Scholar *
>
> *Department of Metallurgical and Materials Engineering Indian Institute of
> Technology Kharagpur, India*
>
>
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