# [Pw_forum] Allowed symmetries in GIPAW

Davide Ceresoli davide.ceresoli at cnr.it
Sat Jul 8 15:41:32 CEST 2017

```Dear Joseph,
that operation is allowed because x=>x, y=>-y, z=>-z,
if I read correctly. The cartesian matrices can be off
diagonal. For example the rotation of 90° along z, is also
allowed, because the triplet (x,y,z) => (-y,x,z) is still
parallel to the original.

Basically the problems are with hexagonal crystals and
with system with very low symmetry.

For hexagonal crystals, it's better to use ibrav=0, and
set a,b in the xy plane, c//z, a forming an angle of 30°
with x.

In all other cases, if unsure, set nosym=.true. in the SCF
calc.

Best wishes,
Davide

On 07/07/2017 12:35 PM, J. Nelson wrote:
> Dear all QE users,
>
> I am using QE-6.1.
>
> The user guide for GIPAW explains that 'symmetry operations that do not map
> cartesian axes are not allowed (i.e. 120 deg. rotations)'.
>
> Does this mean that the Cartesian rotation matrices for each symmetry operation
> - e.g.:
>
>       isym =  4     180 deg rotation - cart. axis [1,0,0]
>
>  cryst.   s( 4) = (     0          1          0      )
>                   (     1          0          0      )
>                   (     0          0         -1      )
>
>  cart.    s( 4) = (  1.0000000  0.0000000  0.0000000 )  [<--this matrix here]
>                   (  0.0000000 -1.0000000  0.0000000 )
>                   (  0.0000000  0.0000000 -1.0000000 )
>
> need to be diagonal for the symmetry operation to be compatible with GIPAW? Are
> symmetry operations with an associated fractional translation allowed?
>
>
> Many thanks,
>
> Joseph Nelson
> TCM Group
> Cavendish Laboratory
> University of Cambridge
>

--
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Davide Ceresoli
CNR Institute of Molecular Science and Technology (CNR-ISTM)
c/o University of Milan, via Golgi 19, 20133 Milan, Italy
Email: davide.ceresoli at cnr.it
Phone: +39-02-50314276, +39-347-1001570 (mobile)
Skype: dceresoli