[Pw_forum] dismatch of Gamma phonon frequencies from different way

[Paolo Giannozzi]

On Wed, 2015-03-18 at 21:31 +0800, 黄志硕 wrote:

> when I calculated the phonon-dispersion based on a q-mesh of 4*4*1,
> after q2r.x and matdyn.x calculation, the Gamma point phonon frequency
> in the "matdyn.modes" file differed a lot:

actually they are quite exactly the same, with the exception of freq(4)
that is not doubly degenerate and of an additional freq(7) that is not
found in the first set of calculations (how did you obtain them, by
the way?). This looks like a bug in symmetrization. Please file a bug
report on qe-forge, or provide an input that can be run

Paolo

>      freq (    1) = -0.000007 [cm-1]
>      freq (    2) =-0.000006 [cm-1]
>      freq (    3) = -0.000001 [cm-1]
>      freq (    4) = 176.907415 [cm-1]
>      freq (    5) =269.403108 [cm-1]
>      freq (    6) =269.403108 [cm-1]
>      freq (    7) = 298.948294 [cm-1]
>      freq (    8) =338.760427 [cm-1]
>      freq (    9) =340.877820 [cm-1].
> The parameters in the input file are: ecutwfc = 90, ecutrho =
> 900, conv_thr =  1.0d-8, mixing_beta = 0.7, etot_conv_thr and
> forc_conv_thr are set default, and the K-mesh is set 17 17 2 1 1 1.
> During phonon calculation, "ASR" is set 'simple'.
> 
> 
> Before calculation, crystal structure was fully relaxed, and the
> convergence of K-mesh and cut-off of plane wave was achieved.
> I also calculated the phonon-dispersion based on q mesh 4*4*2 and
> 8*8*1, and got the same results with that of q mesh 4*4*1.
> 
> 
> I have no idea why the phonon frequencies of Gamma point get from
> single-point calculation and phonon-dispersion calculation differ so
> much. I will appreciate it if anyone could give me some instructions.
> 
> 
> 
> 
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-- 
Paolo Giannozzi, Dept. Chemistry&Physics&Environment, 
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222