[Pw_forum] Star_q degeneracy error

[Stefano de Gironcoli]

ma Yanming wrote:

> ........................
> It seems that in this hcp case, we don't realize what we are supposed
> to realize (obtaining the phonons at any wave vectr). The phonons fail
> at some wave vectors. Is there any good idea to perfect the PWSCF code?
>
- One could use a different definition for the check (based on the
cartesian distance between q and rotated q and not on a
component-by-component check).
- One could increse the acceptance threshold or reduce it.
- One could do the calculation removing the symmetry.

I'm confident that this will never be enough to beat the fantasy of
users in finding out a problematic input.

I remain of my opinion that this IS NOT A BUG.

The code finds correctily that there is a problem in the input ands
signal what it is (albeit in a criptic way): the coordinate are such
that the symmetry check fails because they are just enough different
from an high symmetry point that some symmetry operation is found and
some other not in an inpredictable way. In this situation the code
CORRECTLY STOPS and let you have the opportunity to fix the input or
modifiy the acceptance threshold if you prefere.

But some threshold MUST be specified and therefore THERE ALWAYS BE some
q point that confuses the code no matter how you define the threshold or
the check.
You can only shift the problem away from the specific point you are
considering.

regards,

stefano