[Pw_forum] Star_q degeneracy error

[ma Yanming]

Dear Stefano de Gironcoli 

Thanks for your excellent comments. Yes, the things go just like your 
suggestion.

As you know, before we try to calcuate a whole phonon dispersion curves 
along high symmetry direction in the BZ, the most important thing is to 
determine how many and which special q points we have to calculate directly 
for the selected k mesh (say 444 or 666), then after that by using the 
resulting force constants we can obtain the phonon dispersion. I made my 
own code to choose the special q points. In my case, due to the numerical 
error (from my input) instead of (0.50000000 0.288675135 0.0000000), I got 
the (0.49999999 0.28868359 0.00000000). The discrepancy in y axis is 
8x10-6. Actually, in real situation, the difference in phonon for these two 
q points is negligible, if the codes go through both the two points. 
The big advantage of linear response theory to calculate the phonons is 
from the reality of obtaining phonons at any wave vector. By this method, 
we can always check how good is the phonons we calculate by the 
interploation of force constant in the BZ. This is also the reason why some 
people stop using the supercell method in the phonon calculation.
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?

Regards!

Yanming Ma PhD
Steacie Institute for Molecular Sciences,
National Research Councils of Canada.
100 Sussex
K1A 0R6





>From: Stefano de Gironcoli <degironc at sissa.it>
>Reply-To: pw_forum at pwscf.org
>To: pw_forum at pwscf.org
>Subject: Re: [Pw_forum] Star_q degeneracy error
>Date: Sat, 13 Mar 2004 19:19:47 +0100 (CET)
>
>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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