On 07/19/2011 09:46 AM, 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?
Of course, you can, but be sure this is refer to the E-V plot. > > > 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 I don't have experience on such thing. Perhaps others can help. Regards > > 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 -- Hongsheng Zhao <zhaohscas at yahoo.com.cn> School of Physics and Electrical Information Science, Ningxia University, Yinchuan 750021, China