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

[Éric Germaneau]

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 can neglect 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 <mailto: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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>
>
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-- 
/Be the change you wish to see in the world
/ --- Mahatma Gandhi ---

Dr. Éric Germaneau 
<http://www.linkedin.com/pub/%C3%A9ric-germaneau/30/931/986>

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

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