[Pw_forum] Star_q degeneracy error

Stefano de Gironcoli degironc at sissa.it
Sat Mar 13 19:19:47 CET 2004

dear Yanming Ma,
in your script you are using the following ccordinates
q = (0.49999999 0.28868359 0.00000000)
I guess you want to study the K point in the BZ
q = (0.5, 0.5/sqrt(3.0), 0.0) = 
    (0.50000000 0.288675135 0.0000000)

This means that the q-point you are using is NOT the 
high symmetry K point by an amount of the order of
8.455 x 10^-6 (on the y coordinate).

In order to check whether a symmetry operation belongs
to the small-group of q the code performs a comparison 
between q and the rotated q having an aceptance tollerance 
that is fixed as 1.0x10-5 (in routine PW/eqvect.f90) which
is very close to the accuracy you used to specify the K point.
Probably the symmetry check is passed for some symmetry 
operations and not in some other cases in a way that confuses
the code.

I would say that it is not a bug: some acceptance threshold in
the symmetry check need to be set and if the accuracy used in 
the coordinates is borderline the code my not figure out properly 
the symmetry and then it MUST stop and complain.

Use more accurate values for the coordinates... it works !

Stefano de Gironcoli 

On Fri, 12 Mar 2004, ma Yanming wrote:

> Dear Paolo,
> The error occurred when I used both the 1.2.0 and the lastest 2.0 versions.
> I also tried for different PP. It didn't help.
> My case is Si (metal phase) with simple hcp structure (space group is 
> P6/mmm) a=4.7599 a.u., c=0.9365.
> This "star_q wrong degeneracy " error only occurs for several q points. I 
> read the source code about this error. It is related to the crystal 
> symmetry. I couldn't figure out what is the reason for this error in my 
> case. I attached my run_scripts file in the attachment. Please have a look.
> Thanks 
> Yanming Ma PhD
> Steacie Institute for Molecular Sciences,
> National Research Councils of Canada.
> 100 Sussex
> K1A 0R6
> >From: Paolo Giannozzi <giannozz at nest.sns.it>
> >Reply-To: pw_forum at pwscf.org
> >To: pw_forum at pwscf.org
> >Subject: Re: [Pw_forum] Star_q degeneracy error
> >Date: Wed, 10 Mar 2004 19:54:31 +0100
> >
> >On Wednesday 10 March 2004 19:04, ma Yanming wrote:
> >
> > > "Star_q degeneracy error".
> >
> >very nice error indeed. Which version of the code are you using?
> >If it isn't the last, could you please verify if the error is still there 
> in
> >the last version? Otherwise, please post enough input data to
> >reproduce the error
> >
> >Paolo
> >
> >--
> >Paolo Giannozzi             e-mail:  giannozz at nest.sns.it
> >Scuola Normale Superiore    Phone:   +39/050-509876, Fax:-563513
> >Piazza dei Cavalieri 7      I-56126 Pisa, Italy
> >
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