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

Éric Germaneau germaneau at gucas.ac.cn
Mon Oct 10 20:11:58 CEST 2011


Laptop?! That's my me the point ...
Also usually people  use a  supercell to compute phonon.

On 10/10/2011 01:47 AM, GAO Zhe wrote:
> 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 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
>
>     /Please, if possible, don't send me MS Word or PowerPoint attachments
>     Why? See: http://www.gnu.org/philosophy/no-word-attachments.html /
>
>
>
>
>
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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

/Please, if possible, don't send me MS Word or PowerPoint attachments
Why? See: http://www.gnu.org/philosophy/no-word-attachments.html /

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