[Pw_forum] Pressure-induced phase transition

[Hongsheng Zhao]

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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-- 
Hongsheng Zhao <zhaohscas at yahoo.com.cn>
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China