On 07/19/2011 10:21 PM, Hongsheng Zhao wrote: > On 07/20/2011 09:24 PM, Eric Germaneau wrote: >> Well, I don't know about "common tangent" concept. > See the page 3 of this notes by Keith Refson: > > http://www.castep.org/CASTEP_talks_07/refson2.pdf Thank you. > > >> You can study structure stability looking at phonon dispersion for instance. >> An instability could mean that the structure undergoes a phase >> transition (or that it's not fully relaxed) > What's the characteristic of the phonon dispersion when phase transition > occurs? More hints? I noticed the reply of Pascal Boulet after I sent my post. He point it out very well. > >> You can also compute the free energy of your system where phonons will >> give you the harmonic part (see PRL 64 045123 (2001) >> <http://link.aps.org/doi/10.1103/PhysRevB.64.045123>). >> So you can look at several thing when changing the pressure, among them >> structure change, phonon, energy, free energy (not that easy). >> Why not electronic density of sates? > Again, do you have experience in using DOS for judging phase transition? > Any hints on the the characteristic of it when the phase transition > occurs? I've no idea, that was just a suggestion. I suppose some change may occur in the electronic structure ... But the effect of the pressure on the electronic structure may be less significant than the temperature. I don't know. An expert on /ab initio/ calculation would be able to give you an answer. > > Regards >> Good luck, >> >> ?ric. >> >> On 07/18/2011 09:46 PM, Amin Torabi wrote: >>> Thanks Eric and Hongsheng for the discussion! >>> >>> Can I also have your comments on this "common tangent" concept in >>> first order phase transitions? Is it applicable for my case? >>> >>> >>> And how can I use phonon calculations to help me find the phase >>> transition? I've heard about some "mode softening", but I am not sure >>> how it can be used in this context? Could you refer me to some article >>> explaining the theory behind this >>> >>> Thanks again >>> Amin >>> >>> >>> >>> >>> >>> On Mon, Jul 18, 2011 at 9:28 PM, Hongsheng Zhao >>> <zhaohscas at yahoo.com.cn<mailto:zhaohscas at yahoo.com.cn>> wrote: >>> >>> On 07/19/2011 06:49 PM, Eric Germaneau wrote: >>> > >>> > >>> > On 07/18/2011 10:45 AM, Hongsheng Zhao wrote: >>> >> On 07/19/2011 09:23 AM, Eric Germaneau wrote: >>> >>> Hey Amin, >>> >>> >>> >>> The approach you proposed is ok but you should relax the cell >>> as well. >>> >>> By keeping the cell fix the system does not feel the pressure >>> you wish >>> >>> to apply. >>> >> Not so clear about the meaning of this sentence, could you >>> please give >>> >> some more explanations? ;-( >>> > Under pressure keeping the unit cell fix is useless since the system >>> > won't feel it. >>> > It just makes sense. >>> > So, if you wish to see the effect of the pressure you have to >>> perform >>> > vc-relax calculation. >>> > Take it that way, in thermodynamic you have 2 important ensembles, >>> > isochoric and isobaric. >>> > So either the volume is fix and you get out the corresponding >>> pressure, >>> > or the pressure is fix and you get out the corresponding volume. >>> > Note that the volume of gas for instance decreases inversely >>> > proportionally to pressure. >>> > The pressure is the intensive variable related to the volume >>> (extensive). >>> > You simply can not have both of them constant at the same time. >>> >>> Yes, you're right. I'm so sorry for my previous misleading posts on >>> this thread. Thanks again. >>> >>> > Hope it helps. >>> >> >>> >>> You can also change the volume, relax the positions only and >>> compute the >>> >>> pressure. >>> >>> The best is to compute the free energy as function of pressure >>> ans so >>> >>> the QHA method is the best way to go. >>> >> I think this method cann't do the job of >>> structure-searching-and-finding >>> >> for unknown phases when we changing the external pressure. >>> > Yes, it can but in the case of structure-searching it's very >>> expensive. >>> > In that case you'd better generate random unit cell parameters, >>> random >>> > positions and run vc-relax. >>> > Even this can be expensive since you have to do it a lot of >>> times .... >>> > Just take a look to this papers, PRL 97 1 (2006) >>> > <http://link.aps.org/doi/10.1103/PhysRevLett.97.045504> and JPCM 23 >>> > 053201 (2011) >>> > >>> >>> <http://stacks.iop.org/0953-8984/23/i=5/a=053201?key=crossref.44087776832bc9a0edd0dc492a67d376>. >>> > It is the main work of Professor Pickard >>> > >>> >>> <http://www.st-andrews.ac.uk/physics/condmat/pickard/Crystals%20ex%20nihilo.html>. >>> > Best. >>> >>> Thanks a lot for this information ;-) >>> >>> Regards >>> > >>> >> >>> >> Regards >>> >> >>> >>> Note that at first you can ignore the thermal effect and >>> compute the >>> >>> free energy from a regular phonon calculations. >>> >>> I'm also very interested in such calculations but don't have >>> experiences >>> >>> doing this by mean of first principal calculations. >>> >>> >>> >>> -- >>> Hongsheng Zhao<zhaohscas at yahoo.com.cn >>> <mailto:zhaohscas at yahoo.com.cn>> >>> School of Physics and Electrical Information Science, >>> Ningxia University, Yinchuan 750021, China >>> _______________________________________________ >>> Pw_forum mailing list >>> Pw_forum at pwscf.org<mailto:Pw_forum at pwscf.org> >>> http://www.democritos.it/mailman/listinfo/pw_forum >>> >>> >>> >>> >>> -- >>> Amin Torabi >>> Ph.D. Student >>> Chemistry Department >>> /The/ University /of/ Western Ontario >>> London, On Canada, N6A 5B7 >>> Phone: 519-661-2111 Ext: 87871 >>> **************************************** >>> >>> >>> >>> _______________________________________________ >>> 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<mailto:germaneau at gucas.ac.cn> >> >> College of Physical Sciences >> Graduate University of Chinese Academy of 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> College of Physical Sciences Graduate University of Chinese Academy of 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/20110720/c7be6d8a/attachment-0001.htm
