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On 07/19/2011 10:21 PM, Hongsheng Zhao wrote:
<blockquote cite="mid:4E263BBB.5080705@yahoo.com.cn" type="cite">
<pre wrap="">On 07/20/2011 09:24 PM, Eric Germaneau wrote:
</pre>
<blockquote type="cite">
<pre wrap="">Well, I don't know about "common tangent" concept.
</pre>
</blockquote>
<pre wrap="">
See the page 3 of this notes by Keith Refson:
<a class="moz-txt-link-freetext" href="http://www.castep.org/CASTEP_talks_07/refson2.pdf">http://www.castep.org/CASTEP_talks_07/refson2.pdf</a></pre>
</blockquote>
<font color="#993399">Thank you.</font><br>
<blockquote cite="mid:4E263BBB.5080705@yahoo.com.cn" type="cite">
<pre wrap="">
</pre>
<blockquote type="cite">
<pre wrap="">You can study structure stability looking at phonon dispersion for instance.
An instability could mean that the structure undergoes a phase
transition (or that it's not fully relaxed)
</pre>
</blockquote>
<pre wrap="">
What's the characteristic of the phonon dispersion when phase transition
occurs? More hints?</pre>
</blockquote>
<font color="#993399">I noticed the reply of Pascal Boulet after I
sent my post. He point it out very well.</font><br>
<blockquote cite="mid:4E263BBB.5080705@yahoo.com.cn" type="cite">
<pre wrap="">
</pre>
<blockquote type="cite">
<pre wrap="">You can also compute the free energy of your system where phonons will
give you the harmonic part (see PRL 64 045123 (2001)
<a class="moz-txt-link-rfc2396E" href="http://link.aps.org/doi/10.1103/PhysRevB.64.045123"><http://link.aps.org/doi/10.1103/PhysRevB.64.045123></a>).
So you can look at several thing when changing the pressure, among them
structure change, phonon, energy, free energy (not that easy).
Why not electronic density of sates?
</pre>
</blockquote>
<pre wrap="">
Again, do you have experience in using DOS for judging phase transition?
Any hints on the the characteristic of it when the phase transition
occurs?</pre>
</blockquote>
<font color="#993399">I've no idea, that was just a suggestion.<br>
I suppose some change may occur in the electronic structure ...</font><br>
<font color="#993399">But the effect of the pressure on the</font> <font
color="#993399">electronic structure may be less significant than
the temperature. I don't know.<br>
An expert on <i>ab initio</i> calculation would be able to give
you an answer.<br>
</font>
<blockquote cite="mid:4E263BBB.5080705@yahoo.com.cn" type="cite">
<pre wrap="">
Regards
</pre>
<blockquote type="cite">
<pre wrap="">Good luck,
Éric.
On 07/18/2011 09:46 PM, Amin Torabi wrote:
</pre>
<blockquote type="cite">
<pre wrap="">Thanks Eric and Hongsheng for the discussion!
Can I also have your comments on this "common tangent" concept in
first order phase transitions? Is it applicable for my case?
And how can I use phonon calculations to help me find the phase
transition? I've heard about some "mode softening", but I am not sure
how it can be used in this context? Could you refer me to some article
explaining the theory behind this
Thanks again
Amin
On Mon, Jul 18, 2011 at 9:28 PM, Hongsheng Zhao
<<a class="moz-txt-link-abbreviated" href="mailto:zhaohscas@yahoo.com.cn">zhaohscas@yahoo.com.cn</a> <a class="moz-txt-link-rfc2396E" href="mailto:zhaohscas@yahoo.com.cn"><mailto:zhaohscas@yahoo.com.cn></a>> wrote:
On 07/19/2011 06:49 PM, Eric Germaneau wrote:
>
>
> On 07/18/2011 10:45 AM, Hongsheng Zhao wrote:
>> On 07/19/2011 09:23 AM, Eric Germaneau wrote:
>>> 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.
>> Not so clear about the meaning of this sentence, could you
please give
>> some more explanations? ;-(
> 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.
Yes, you're right. I'm so sorry for my previous misleading posts on
this thread. Thanks again.
> Hope it helps.
>>
>>> 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.
>> I think this method cann't do the job of
structure-searching-and-finding
>> for unknown phases when we changing the external pressure.
> 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.
Thanks a lot for this information ;-)
Regards
>
>>
>> Regards
>>
>>> 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.
--
Hongsheng Zhao <<a class="moz-txt-link-abbreviated" href="mailto:zhaohscas@yahoo.com.cn">zhaohscas@yahoo.com.cn</a>
<a class="moz-txt-link-rfc2396E" href="mailto:zhaohscas@yahoo.com.cn"><mailto:zhaohscas@yahoo.com.cn></a>>
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China
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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="">
--
/Be the change you wish to see in the world
/ — Mahatma Gandhi —
Dr. Éric Germaneau <a class="moz-txt-link-rfc2396E" href="mailto:germaneau@gucas.ac.cn"><mailto:germaneau@gucas.ac.cn></a>
College of Physical Sciences
Graduate University of Chinese Academy of Sciences
Yuquan Road 19A
Beijing 100049
China
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<a
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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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