[Pw_forum] Pressure-induced phase transition

[Eric Germaneau]

On 07/18/2011 08:15 PM, Hongsheng Zhao wrote:
> On 07/19/2011 12:45 AM, Amin Torabi wrote:
>> Dear Hongsheng,
>>
>> I don't understand why I should use relax instead of vc-relax. As far as
>> I understand, when a system is under pressure, both the atomic positions
>> inside a unit cell, as well as the dimensions of the unit cell changes.
>> So, why should I keep the unit cell parameters fixed if I am interested
>> in finding phase transitions?
> In fact, you can do relax or  vc-relax, it depend on the actual ways
> used by you to study the   Pressure-induced phase transition.  Let me
> give you a more detailed description as follows:
>
> 1-  If you only use one type of initial phase to do this type of study.
>    Then you should use the vc-relax when changing the external pressure.
>    And then see what's the ultimate unit cell looks like for each
> pressure step.  For convenience, you can also plot the
> binding-energy/formation-energy vs external-pressure to see whether
> there are some abrupt change points - of course, there are many other
> physical properties you can consider to assist you judging the first
> order phase transition, I just give you a example here - on the plot.
>
> 2-  If you have two or more  initial phases and want to find the phase
> transition pressure among them.  In this case, you can do the relax
> calculation for each phase  by impose external pressure in certain
> range.  And then you can plot the E-V or H-V plots for each phase in the
> same Cartesian system  to find the  phase transition point.
>
> Basically, the first method (vc-relax) enable you to find or predict
> some possible phase you don't known in advance.  The second method
> (relax) enable you find the phase transition pressure for known phases.
>
> But, even by using the vc-relax, I don't think it can be served as a
> perfect method for new-phase finding an searching technology.  This is
> because the vc-relax just changing the unit cells ( cell parameters and
> atoms positions ) in a series of localized positions.  So I don't think
> by using this method you can do the job like universal structure
> prediction just as the uspex or similar codes done for you.
They use the free energy and so it's more accurate.
The cell relaxation procedure can give good results (See the work of 
Prof. Pickard).
Anyway, I don't want to debate who is right or wrong. I simply don't 
believe in that.
The guy wanted our opening, I got it.
Best,

            Éric.
>
> Regards
>> Thanks
>

-- 
/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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