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On 07/18/2011 08:15 PM, Hongsheng Zhao wrote:
<blockquote cite="mid:4E24CC8B.4050407@yahoo.com.cn" type="cite">
<pre wrap="">On 07/19/2011 12:45 AM, Amin Torabi wrote:
</pre>
<blockquote type="cite">
<pre wrap="">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?
</pre>
</blockquote>
<pre wrap="">
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.</pre>
</blockquote>
<font color="#663366">They use the free energy and so it's more
accurate.<br>
The cell relaxation procedure can give good results (See the work
of Prof. Pickard).<br>
Anyway, I don't want to debate who is right or wrong. I simply
don't believe in that.<br>
The guy wanted our opening, I got it.<br>
Best,<br>
<br>
Éric.</font><br>
<blockquote cite="mid:4E24CC8B.4050407@yahoo.com.cn" type="cite">
<pre wrap="">
Regards
</pre>
<blockquote type="cite">
<pre wrap="">
Thanks
</pre>
</blockquote>
<pre wrap="">
</pre>
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<em>
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— Mahatma Gandhi —<br>
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<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>
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