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On 07/18/2011 09:28 PM, Hongsheng Zhao wrote:
<blockquote cite="mid:4E24DDD1.5040603@yahoo.com.cn" type="cite">
<pre wrap="">On 07/19/2011 06:49 PM, Eric Germaneau wrote:
</pre>
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<pre wrap="">
On 07/18/2011 10:45 AM, Hongsheng Zhao wrote:
</pre>
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<pre wrap="">On 07/19/2011 09:23 AM, Eric Germaneau wrote:
</pre>
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<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>
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<pre wrap="">Under pressure keeping the unit cell fix is useless since the system
won't feel it.
It just makes sense.
So, if you wish to see the effect of the pressure you have to perform
vc-relax calculation.
Take it that way, in thermodynamic you have 2 important ensembles,
isochoric and isobaric.
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.
Note that the volume of gas for instance decreases inversely
proportionally to pressure.
The pressure is the intensive variable related to the volume (extensive).
You simply can not have both of them constant at the same time.
</pre>
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<pre wrap="">
Yes, you're right. I'm so sorry for my previous misleading posts on
this thread. Thanks again.</pre>
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<font color="#000066">No worries!<br>
I'm glad to see that this discussion was useful.<br>
Best,<br>
<br>
Éric.</font><br>
<blockquote cite="mid:4E24DDD1.5040603@yahoo.com.cn" type="cite">
<pre wrap="">
</pre>
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<pre wrap="">Hope it helps.
</pre>
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<pre wrap="">
</pre>
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<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>
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<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>
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<pre wrap="">Yes, it can but in the case of structure-searching it's very expensive.
In that case you'd better generate random unit cell parameters, random
positions and run vc-relax.
Even this can be expensive since you have to do it a lot of times ....
Just take a look to this papers, PRL 97 1 (2006)
<a class="moz-txt-link-rfc2396E" href="http://link.aps.org/doi/10.1103/PhysRevLett.97.045504"><http://link.aps.org/doi/10.1103/PhysRevLett.97.045504></a> and JPCM 23
053201 (2011)
<a class="moz-txt-link-rfc2396E" href="http://stacks.iop.org/0953-8984/23/i=5/a=053201?key=crossref.44087776832bc9a0edd0dc492a67d376"><http://stacks.iop.org/0953-8984/23/i=5/a=053201?key=crossref.44087776832bc9a0edd0dc492a67d376></a>.
It is the main work of Professor Pickard
<a class="moz-txt-link-rfc2396E" href="http://www.st-andrews.ac.uk/physics/condmat/pickard/Crystals%20ex%20nihilo.html"><http://www.st-andrews.ac.uk/physics/condmat/pickard/Crystals%20ex%20nihilo.html></a>.
Best.
</pre>
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<pre wrap="">
Thanks a lot for this information ;-)
Regards
</pre>
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<pre wrap="">
</pre>
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<pre wrap="">
Regards
</pre>
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<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.
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</pre>
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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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