[Pw_forum] Pressure-induced phase transition

[Hongsheng Zhao]

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
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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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-- 
Hongsheng Zhao <zhaohscas at yahoo.com.cn>
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China