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Hey Amin,<br>
<br>
The approach you proposed is ok but you should relax the cell as
well.<br>
By keeping the cell fix the system does not feel the pressure you
wish to apply.<br>
You can also change the volume, relax the positions only and compute
the pressure.<br>
The best is to compute the free energy as function of pressure ans
so the QHA method is the best way to go.<br>
Note that at first you can ignore the thermal effect and compute the
free energy from a regular phonon calculations.<br>
I'm also very interested in such calculations but don't have
experiences doing this by mean of first principal calculations.<br>
<br>
I hope that helps,<br>
Good luck,<br>
<br>
Éric.<br>
<br>
On 07/18/2011 04:04 AM, Hongsheng Zhao wrote:
<blockquote cite="mid:4E23E910.1050004@yahoo.com.cn" type="cite">
<pre wrap="">On 07/18/2011 09:40 AM, Amin Torabi wrote:
</pre>
<blockquote type="cite">
<pre wrap="">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
</pre>
</blockquote>
<pre wrap="">
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.
</pre>
<blockquote type="cite">
<pre wrap="">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
</pre>
</blockquote>
<pre wrap="">
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
</pre>
<blockquote type="cite">
<pre wrap="">
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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<pre wrap="">
</pre>
</blockquote>
<br>
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Be the change you wish to see in the world<br>
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<small>
<font color="#946e8c">
— Mahatma Gandhi —<br>
</font></small>
<br>
<a href="mailto:germaneau@gucas.ac.cn">Dr. Éric Germaneau</a><br>
<br>
College of Physical Sciences<br>
Graduate University of Chinese Academy of Sciences<br>
Yuquan Road 19A<br>
Beijing 100049<br>
China<br>
<br>
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