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

[GAO Zhe]

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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