[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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> --
> /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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