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

[GAO Zhe]

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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