[Pw_forum] Pressure-induced phase transition

[Eric Germaneau]

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.
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.
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,

                               Éric.

On 07/18/2011 04:04 AM, Hongsheng Zhao wrote:
> On 07/18/2011 09:40 AM, Amin Torabi wrote:
>> Dear experts!
>>
>> Forgive me for my naive question... but I am interested in
>> pressure-induced phase transitions of diborane. Could you please have a
>> look at my input file, attached to this message.
>>
>> The following is what I think I should do: (correct me if I'm wrong!)
>>
>> 1. doing separate vc-relax calculation at each pressure
> By using this method, there are some limitations.  You should have known
> in advance the different phases between them the phase transition maybe
> occur when we change the pressure.  Then you can use this method to
> determine the phase transition path.  But I'm not so sure this method
> can predict all of the possible phases for a system when we change the
> external pressure.  To do a more reasonable and complete phase
> transition prediction and even searching for unknown phases, you should
> use some third-party code like uspex for this type of job.
>
> In addition, I think you should do relax, not vc-relax calculation at
> each pressure.  If you do vc-relax, then the pressure imposed to the
> system will be released.  So you won't get the *Pressure-induced* phase
> transition.
>
>> 2. comparing the optimized cell_parameters at each pressure to see when
>> the structure changes
>>
>> I am wondering how I can use the enthalpy or unit cell volume at each
>> pressure to make sure that a phase transition has occurred
> For the other questions, see this thread on this list:  About the method
> to obtain the common tangent for determing the phase transition
> pressure.  HTH.
>
> Regards
>> Thanks in advance
>>
>>
>> --
>> 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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-- 
/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 /

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