[Pw_forum] Is it possible to calculate free energy of graphite at specified temperature?

[GAO Zhe]

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,"Éric 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.

                                                              Éric.

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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Dr. Éric Germaneau

Graduate University of Chinese Academy of Sciences
College of Physical Sciences
Yuquan Road 19A
Beijing 100049
China

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