[Pw_forum] A strange problem in phonon calculation

[Wei-Bing Zhang]

Dear Dr. Giannozzi and Le,

Thanks very much for your reply.

Actually, I also performed some test calculations using different
q-mesh from 3x3x1 to 8x8x1. It is quite strange again that the
calculation using odd q points i.e. 3x3x1, and 5x5x1 don’t include any
negative phonon frequency. While the calculations with even q-mesh
including 4x4x1, 6x6x1 and 8x8x1 give negative frequencies at q points
near gamma. It should be pointed out that with increasing q-mesh i.e.
from 4x4x1 to 8x8x1; the points with negative frequency become fewer
and the frequency seems to be more close to zero.  However, when I
calculated the phonon frequency at point near Gamma directly, the
negative frequency can be also found in acoustic mode ZA.  I also
tested the calculations using different version of QE (SVN, 4.3.2,
5.0.3 with May 6 patch), all version of codes give the same results.

I am really confused about the results. The results using different
q-mesh are enclosed.  Can I trust the results produced by odd q-mesh
like 5x5x1? Any suggestions will be appreciated.



Thanks and best regards



Wei-Bing Zhang
> On 20/06/2013, Duy Le <ttduyle at gmail.com> wrote:
>> Did you perform the test for q-mesh? I would do 2x2x1, 3x3x1 and
>> higher (if it is not that expensive) to see the evolution of
>> phonon-dispersion. Usually, 4x4x1 q-mesh is enough for converged
>> dispersion.
>>
>>
>> ----------------------------------------------------
>> Duy Le
>> Postdoctoral Associate
>> Department of Physics
>> University of Central Florida.
>> Website: http://www.physics.ucf.edu/~dle
>>
>>
>> On Wed, Jun 19, 2013 at 4:45 PM, Paolo Giannozzi
>> <paolo.giannozzi at uniud.it> wrote:
>>> You can make a calculation at a q point that has imaginary
>>> frequencies, verify if you get the same result or if it is
>>> an artifact of Fourier interpolation
>>>
>>> P.
>>>
>>> On Tue, 2013-06-18 at 09:38 +0800, Wei-Bing Zhang wrote:
>>>> Dear all
>>>>
>>>>
>>>> Recently, I am trying to calculate the phonon dispersion of 2D
>>>> materials. I used 4x4x1 q-points in phonon calculation whereas a
>>>> 16x16x1 k-points for self-consistent electronic calculation. The
>>>> direct calculations indicate that all phonons frequencies are above
>>>> zero on the grid of q-vectors including the Gamma point. However, when
>>>> I transformed the dynamical matrices from G- to R-space and calculated
>>>> phonon dispersions using “matdyn.x”, some negative frequencies appear
>>>> in acoustic mode ZA near Gamma point.  Since I am really a new user
>>>> for phonon calculation, I am not sure what happened. The input and
>>>> output file are enclosed. Could you tell me what’s wrong with my
>>>> calculations? Any suggestions will be appreciated.
>>>>
>>>>
>>>>
>>>> Thanks and best regards
>>>>
>>>>
>>>>
>>>> Wei-Bing Zhang
>>>>
>>>>
>>>>
>>>>
>>>> ----------------------------------------------------------------------
>>>> Dr. Wei-Bing Zhang
>>>>
>>>> School of Physics and Electronic Sciences
>>>> Changsha University of Science and Technology,
>>>> Changsha, People’s Republic of China
>>>> Phone : +86 186 73108538
>>>> E-mail: zwb256 at hotmail.com, weibingzhangavh at gmail.com
>>>> ----------------------------------------------------------------------
>>>> _______________________________________________
>>>> Pw_forum mailing list
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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
>>>
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>>
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>
>
> --
> ----------------------------------------------------------------------
> Dr. Wei-Bing Zhang
>
> School of Physics and Electronic Sciences
> Changsha University of Science and Technology,
> Changsha,  People’s Republic of China
> Phone : +86 186 73108538
> E-mail: zhangwb at csust.edu.cn, weibingzhangavh at gmail.com
> ----------------------------------------------------------------------
>


-- 
----------------------------------------------------------------------
Dr. Wei-Bing Zhang

School of Physics and Electronic Sciences
Changsha University of Science and Technology,
Changsha,  People’s Republic of China
Phone : +86 186 73108538
E-mail: zhangwb at csust.edu.cn, weibingzhangavh at gmail.com
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