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,"?ric 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. > > ?ric. > > 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 >> >> >> >> _______________________________________________ >> Pw_forum mailing list >> Pw_forum at pwscf.org <mailto:Pw_forum at pwscf.org> >> http://www.democritos.it/mailman/listinfo/pw_forum >> >> >> >> >> >> >> _______________________________________________ >> Pw_forum mailing list >> Pw_forum at pwscf.org >> http://www.democritos.it/mailman/listinfo/pw_forum > > -- > /Be the change you wish to see in the world > / --- Mahatma Gandhi --- > > Dr. ?ric 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 / > > > > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum -- /Be the change you wish to see in the world / --- Mahatma Gandhi --- Dr. ?ric 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 / -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20111010/deeb0a5d/attachment.htm
