[Pw_forum] symmetry operations in PW

[alexandra.carvalho at epfl.ch]

Dear Manoj,

I do not know about the definitions intrinsic to pwscf,
but in group theory directed opperations are not allways equivalent.
In this case, the rotations about -z and z will be inequivalent
if there is no opperation of the group which reverses the sense of the z axis.

For example, in the T group there are 8 C3 opperations although there
are only four cube diagonals- see Tinkham "Group Theory and Quantum  
mechanics" 2003 pp 58.

Alexandra Carvalho
EPFL Lausanne

Quoting Manoj Srivastava <manoj at phys.ufl.edu>:

> Dear PWSCF users and developers,
>  I was looking into the code where it calculates symmerty operations for
> the group. Now the symmetry operations are defined as ' 90 deg rotation -
> cart. axis [0,0,-1]', &  ' 90 deg rotation - cart. axis [0,0,1]
> I did not understand the point of defining rotation about z and -z
> axis. Is this because of some numerical constraint? Also, there are
> elements with 'theta' and '-theta' rotation in the same group. I was just
> wondering the reason behind this. Any help will be appreciated.
>
> Regards,
> Manoj Srivastava
> Department of Physics
> University of Florida
> Gainesville, FL, USA.
>
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