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

[bhabya sahoo]

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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