Dear Eric,
Thank you very much for your reply.
I haven't calculated graphite, and when I tried to calculate it, I was in
dormitory, at where I cannot download any paper......
After reading your suggestion, I searched a paper publised at Solid State
Communications 131, 141-152, it clearly showed that the phonon dispersion of
graphite is stable.
I think I may use too small cut-off and k-points due to my poor performance of
laptop~ I will try it, again, for a better relaxation and result.
--
GAO Zhe
CMC Lab, MSE, SNU, Seoul, S.Korea
At 2011-10-11 01:05:44,"?ric Germaneau" <germaneau at gucas.ac.cn> wrote:
Hey Gao,
Well, I have a naive question/reply.
Why do you get imaginary frequencies?
I'd be very surprised of graphite instability at 0K ...
Is you structure relaxed enough?
This structure has been so much studied, have you ever read such graphite
properties before?
May experts can tell us more about it.
?ric.
On 10/10/2011 12:37 AM, GAO Zhe wrote:
Thank you for your kindly reply.
Well, normally, if we found imaginary frequency in phonon dispersion, it
indicates that this structure is not stable or, at least, unstable at 0K.
Therefore, the vibrational free energy becomes unmeaningful because of the
imaginary value.
However, as we know, graphite is stable phase in a wild temperature range,
including 0K. And when we discuss about formation free energy of carbide at
specified temperature, graphite is used as reference pure material instead of
diamond. This means that the free energy of graphite really exists.
Therefore, I wondered whether there are some methods, for example, improving
the graphite model, can deal with this problem and calculate vibrational free
energy of graphite. Or, whether it is possible that we canneglect the vibration
including z-direction and only consider vibration in xy-plane.
--
GAO Zhe
CMC Lab, MSE , SNU, Seoul, S.Korea
At 2011-10-09 20:52:02,"bhabya sahoo" <bdslipun at gmail.com> wrote:
no because it is not valid for negative frequncy in phonon spectrum
2011/10/9 GAO Zhe<flux_ray12 at 163.com>
Dear QE developer and users:
I am using QE 4.3.2 to calculate phonon and thermaldynamis properties of
materials. These days, I need to find the Gibbs (or Helmholtz) Free energy of
carbon. Thus, I compared diamond and graphite phases at 0K. As a result,
graphite provides lower energy per carbon atom (about 0.3eV/atom lower than of
diamond).
However, the phonon dispersion result shows that the imaginary frequency
occurred at Gamma, H, A and K, so it is impossible by using QHA to find the
vibrational free energy at specified temperature.
The graphite model I used is:
ibrav = 0 ,
celldm(1) = 1.889725989 ,
CELL_PARAMETERS
2.130421558318710 -1.229999460182690 0.000000000000000
0.000000000000000 2.459998920365380 0.000000000000000
0.000000000000000 0.000000000000000 6.800000000000000
ATOMIC_POSITIONS crystal
C 0.0000000000000000 0.0000000000000000 0.2500000000000000
C 0.0000000000000000 0.0000000000000000 0.7500000000000000
C 0.3333333333333330 0.6666666666666669 0.2500000000000000
C -0.3333333333333330 -0.6666666666666669 0.7500000000000000
Is there any other methods or models can calculate the free energy of graphite
at specified temperature?
Any suggestion will be welcome.
Thanks.
--
GAO Zhe
CMC Lab, MSE, SNU, Seoul, S.Korea
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Be the change you wish to see in the world
? Mahatma Gandhi ?
Dr. ?ric Germaneau
Graduate University of Chinese Academy of Sciences
College of Physical Sciences
Yuquan Road 19A
Beijing 100049
China
Please, if possible, don't send me MS Word or PowerPoint attachments
Why? See:http://www.gnu.org/philosophy/no-word-attachments.html
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