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I am trying to understand the use of space_group in Quantum Espresso. I note that the keyword can be used with and without ibrav. What is the benefit/usage of this keyword? Is there a performance related reason to give the input only the inequivalent atomic positions?

For reference (and from the documentation):

space_group INTEGER
Default:    0

The number of the space group of the crystal, as given in the International Tables of Crystallography A (ITA). This allows to give in input only the inequivalent atomic positions. The positions of all the symmetry equivalent atoms are calculated by the code. Used only when the atomic positions are of type crystal_sg. See also uniqueb, origin_choice, rhombohedral

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    $\begingroup$ If you have two input files for a system, then try with dry run (setting nstep=0 in the &CONTROL namelist). Then inspect both of the output files, especially note how many inequivalent k-points there are. The more k-points the code needs to sample the less symmetry has been utilized, i.e. the calculation will take longer. Choose the option that gives you less k-points. It is analogous to the k-grid shifting I believe. Shifting k-grid might reduce the number of inequivalent k-points and hence the same accuracy with lower time. $\endgroup$ Feb 22 at 2:14

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The documentation is straightforward on the meaning of this tag, it is used to specifiy the space group of the crystal you are working on in accordance with the well known ITA.

Suppose that you are using space_group without ibrav, then you just need to provide the positions of atoms that has no symmetry operations or not related to symmetry operations, then QE will calculate the positions of the other atoms that has symmetry operations. As an example, suppose you have crystal with space group (Fm-3m):

&control
   ...
/

&system
   ibrav = 0
   space_group = 225
   ...
/

&electrons
   ...
/

ATOMIC_SPECIES
   ...
/

ATOMIC_POSITIONS crystal_sg
   AtomType 0.0 0.0 0.0
   AtomType 0.5 0.5 0.5
/

K_POINTS automatic
   2 2 2 0 0 0
/

When it comes to performance, theoretically speaking, exploiting the symmetry shall give you an acceleration in computional performance, but I am not aware whether it is a notable difference or not, I don't have actual numerical comparison in this regard. You may do your own testing.

Moreover, it was mentioned by Paolo Giannozzi et. al. in QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials, that exploiting the symmetry has an impact on accelerating the computation specially for Phonon calculations.

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  • $\begingroup$ Could you also suggest how to see the space group from the QE output file? I mean which output file and tag can be searched to see the space group (provided that only ibrav has been used but without the space_group tag option in the input)? @Jaafar Mehrez $\endgroup$
    – Sak
    Mar 22 at 1:03

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