[Pw_forum] Pressure-induced phase transition

[Hongsheng Zhao]

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>
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China