[Pw_forum] Pressure-induced phase transition

[Eric Germaneau]

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.
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.

>
> 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.
>>
>> I hope that helps,
>> Good luck,
>

-- 
/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

/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://lists.quantum-espresso.org/pipermail/users/attachments/20110719/445373d7/attachment.html>