I guess it has to do with system having less symmetry than expected from the Bravais lattice.

This link might be of help.

https://www.mail-archive.com/[email protected]/msg37218.html

Michael Tekle Thanks for the response. But i don't find any solution.

Hello Aiswarya Priyambada

This problem happens because input files and atomic positions in Quantum ESPRESSO is a little confusing. First, the fact you set ibrav=8, does NOT guarantee that Quantum ESPRESSO will understand the symmetry it as it should be. The atomic positions is the most important part, so, maybe your input file must be corrected to give the correct atomic positions that lead to the symmetry you want. If you need more detailed explanation, please let me know.

Best regards,

Ricardo Tadeu

Ricardo Tadeu Maia Thanks for your response. Can you please explain it more elaborately?

Thanks

Aiswarya Priyambada, I will give you this example I was dealing with. I was doing calculations with Beryllium, which is a hexagonal structure. At first, I was trying to calculate elastic constants using this input: "SCF_incorrect_D2h.in", thinking that it was a good start to thermo_pw. But the problem was that QE was not considering that this structure was hexagonal. I got this error:

ibrav=0 or Bravais lattice not compatible with the point group.

The code will not use symmetry.

Cannot use the Laue class with ibrav=0, using laue=0

I will use elastic constants with the form

Laue class not programmed or ibrav=0 using C_i

And I struggled a lot to understand why, because ibrav=4 was set, and the atomic positions for 2 beryllium atoms were right, as you can see check in the input:

...

&SYSTEM

ibrav=4

A=3.181736338

C=2.271292659

nat=2

ntyp=1

ecutwfc=40.0

ecutrho=240.0

occupations='smearing'

degauss=0.02

smearing='gaussian'

...

ATOMIC_POSITIONS (crystal)

Be 0.667 0.333 0.75 1 1 1

Be 0.333 0.667 0.25 1 1 1

...

The problem was that QE was understanding "0.667" like "0.667000", and "0.333" was "0.333000". Thus the symmetry is not hexagonal if you consider this. So, in the "SCF_correct_D6h.in" I set the coordinates like:

ATOMIC_POSITIONS (crystal)

Be 0.666667 0.333333 0.75 1 1 1

Be 0.333333 0.666667 0.25 1 1 1

And there it was, with this change QE understood the system as hexagonal. You can try to show the complete symmetry of your system in PW, with the command verbosity= 'high':

&CONTROL

outdir='.\outdir'

calculation='scf'

tprnfor=.true.

tstress=.true.

pseudo_dir='.'

verbosity='high'

Check both output files I attached and compare the symmetry and also the time of calculation. Setting the correct symmetry decreased a lot of time. I started PW with this wrong symmetry, and in 50 seconds of calculation it didn't return even a single energy value, while with the correct symmetry, all calculation was done in 5 seconds.

I think that your system has some problem like this, as Mr. Michael Tekle suggested. Please, check if you have this incompatibility in your calculations.

Best regards,

Ricardo Tadeu

Ricardo Tadeu Maia Thanks for your response.