[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
>>> Pw_forum at pwscf.org
>>> http://pwscf.org/mailman/listinfo/pw_forum
>>
>> --
>> Paolo Giannozzi, Dept. Chemistry&Physics&Environment,
>> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
>> Phone +39-0432-558216, fax +39-0432-558222
>>
>> _______________________________________________
>> Pw_forum mailing list
>> Pw_forum at pwscf.org
>> http://pwscf.org/mailman/listinfo/pw_forum
>
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum
>


-- 
----------------------------------------------------------------------
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
----------------------------------------------------------------------
-------------- next part --------------
A non-text attachment was scrubbed...
Name: eps.zip
Type: application/zip
Size: 22380 bytes
Desc: not available
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20130621/c1150110/attachment.zip>