[Pw_forum] Pressure-induced phase transition

[Eric Germaneau]

On 07/18/2011 09:28 PM, Hongsheng Zhao 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.
No worries!
I'm glad to see that this discussion was useful.
Best,

         Éric.
>
>> 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.
>
>

-- 
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/ --- 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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