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On 07/18/2011 10:45 AM, Hongsheng Zhao wrote:
<blockquote cite="mid:4E244713.5060008@yahoo.com.cn" type="cite">
<pre wrap="">On 07/19/2011 09:23 AM, Eric Germaneau wrote:
</pre>
<blockquote type="cite">
<pre wrap="">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.
</pre>
</blockquote>
<pre wrap="">
Not so clear about the meaning of this sentence, could you please give
some more explanations? ;-(</pre>
</blockquote>
<font color="#003300">Under pressure keeping the unit cell fix is
useless since the system won't feel it.<br>
It just makes sense.<br>
So, if you wish to see the effect of the pressure you have to
perform vc-relax calculation.<br>
Take it that way, in thermodynamic you have 2 important ensembles,
isochoric and isobaric.<br>
So either the volume is fix and you get out the corresponding
pressure, or the pressure is fix and you get out the corresponding
volume.<br>
Note that the volume of gas for instance decreases inversely
proportionally to pressure.<br>
The pressure is the intensive variable related to the volume
(extensive). <br>
You simply can not have both of them constant at the same time.<br>
Hope it helps.</font><br>
<blockquote cite="mid:4E244713.5060008@yahoo.com.cn" type="cite">
<pre wrap="">
</pre>
<blockquote type="cite">
<pre wrap="">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.
</pre>
</blockquote>
<pre wrap="">
I think this method cann't do the job of structure-searching-and-finding
for unknown phases when we changing the external pressure.</pre>
</blockquote>
<font color="#003300">Yes, it can but in the case of
structure-searching it's very expensive.<br>
In that case you'd better generate random unit cell parameters,
random positions and run vc-relax.<br>
Even this can be expensive since you have to do it a lot of times
....<br>
Just take a look to this papers, <a
href="http://link.aps.org/doi/10.1103/PhysRevLett.97.045504">PRL
97 1 (2006)</a> and <a
href="http://stacks.iop.org/0953-8984/23/i=5/a=053201?key=crossref.44087776832bc9a0edd0dc492a67d376">JPCM
23 053201 (2011)</a>.<br>
It is the main work of <a
href="http://www.st-andrews.ac.uk/physics/condmat/pickard/Crystals%20ex%20nihilo.html">Professor
Pickard</a>.<br>
Best.</font><br>
<br>
<blockquote cite="mid:4E244713.5060008@yahoo.com.cn" type="cite">
<pre wrap="">
Regards
</pre>
<blockquote type="cite">
<pre wrap="">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,
</pre>
</blockquote>
<pre wrap="">
</pre>
</blockquote>
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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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