[Pw_forum] Pressure-induced phase transition

[Eric Germaneau]

Well, I don't know about "common tangent" concept.
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)
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?
Good luck,

           Éric.

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
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>
>
>
> -- 
> Amin Torabi
> Ph.D. Student
> Chemistry Department
> /The/ University /of/ Western Ontario
> London, On Canada, N6A 5B7
> Phone: 519-661-2111 Ext: 87871
> ****************************************
>
>
>
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-- 
/Be the change you wish to see in the world
/ --- Mahatma Gandhi ---

Dr. Éric Germaneau <mailto:germaneau at gucas.ac.cn>

College of Physical Sciences
Graduate University of Chinese Academy of Sciences
Yuquan Road 19A
Beijing 100049
China

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