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Well, I don't know about "common tangent" concept.<br>
You can study structure stability looking at phonon dispersion for
instance.<br>
An instability could mean that the structure undergoes a phase
transition (or that it's not fully relaxed)<br>
You can also compute the free energy of your system where phonons
will give you the harmonic part (see <a
href="http://link.aps.org/doi/10.1103/PhysRevB.64.045123">PRL 64
045123 (2001)</a>).<br>
So you can look at several thing when changing the pressure, among
them structure change, phonon, energy, free energy (not that easy).<br>
Why not electronic density of sates?<br>
Good luck,<br>
<br>
Éric. <br>
<br>
On 07/18/2011 09:46 PM, Amin Torabi wrote:
<blockquote
cite="mid:CAEXRF5ycO_UN1nMKvbEdBqL7dNv+rwDK0+tSG9LcFHkhpvLnfA@mail.gmail.com"
type="cite">
<div dir="ltr">Thanks Eric and Hongsheng for the discussion!<br>
<br>
Can I also have your comments on this "common tangent" concept
in first order phase transitions? Is it applicable for my case?<br>
<br>
<br>
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<br>
<br>
Thanks again <br>
Amin<br>
<br>
<br>
<br>
<br>
<br>
<div class="gmail_quote">On Mon, Jul 18, 2011 at 9:28 PM,
Hongsheng Zhao <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:zhaohscas@yahoo.com.cn">zhaohscas@yahoo.com.cn</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;">
<div class="im">On 07/19/2011 06:49 PM, Eric Germaneau
wrote:<br>
><br>
><br>
> On 07/18/2011 10:45 AM, Hongsheng Zhao wrote:<br>
>> On 07/19/2011 09:23 AM, Eric Germaneau wrote:<br>
>>> 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<br>
>>> to apply.<br>
>> Not so clear about the meaning of this sentence,
could you please give<br>
>> some more explanations? ;-(<br>
> Under pressure keeping the unit cell fix is useless
since the system<br>
> 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<br>
> vc-relax calculation.<br>
> Take it that way, in thermodynamic you have 2
important ensembles,<br>
> isochoric and isobaric.<br>
> So either the volume is fix and you get out the
corresponding pressure,<br>
> or the pressure is fix and you get out the
corresponding volume.<br>
> Note that the volume of gas for instance decreases
inversely<br>
> 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>
<br>
</div>
Yes, you're right. I'm so sorry for my previous misleading
posts on<br>
this thread. Thanks again.<br>
<div class="im"><br>
> Hope it helps.<br>
>><br>
>>> You can also change the volume, relax the
positions only and compute the<br>
>>> pressure.<br>
>>> The best is to compute the free energy as
function of pressure ans so<br>
>>> the QHA method is the best way to go.<br>
>> I think this method cann't do the job of
structure-searching-and-finding<br>
>> for unknown phases when we changing the external
pressure.<br>
> 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<br>
> 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, PRL 97 1 (2006)<br>
</div>
> <<a moz-do-not-send="true"
href="http://link.aps.org/doi/10.1103/PhysRevLett.97.045504"
target="_blank">http://link.aps.org/doi/10.1103/PhysRevLett.97.045504</a>>
and JPCM 23<br>
> 053201 (2011)<br>
> <<a moz-do-not-send="true"
href="http://stacks.iop.org/0953-8984/23/i=5/a=053201?key=crossref.44087776832bc9a0edd0dc492a67d376"
target="_blank">http://stacks.iop.org/0953-8984/23/i=5/a=053201?key=crossref.44087776832bc9a0edd0dc492a67d376</a>>.<br>
<div class="im">> It is the main work of Professor
Pickard<br>
</div>
> <<a moz-do-not-send="true"
href="http://www.st-andrews.ac.uk/physics/condmat/pickard/Crystals%20ex%20nihilo.html"
target="_blank">http://www.st-andrews.ac.uk/physics/condmat/pickard/Crystals%20ex%20nihilo.html</a>>.<br>
> Best.<br>
<br>
Thanks a lot for this information ;-)<br>
<br>
Regards<br>
<div class="im">><br>
>><br>
>> Regards<br>
>><br>
>>> Note that at first you can ignore the thermal
effect and compute the<br>
>>> free energy from a regular phonon
calculations.<br>
>>> I'm also very interested in such calculations
but don't have experiences<br>
>>> doing this by mean of first principal
calculations.<br>
<br>
<br>
<br>
</div>
<div class="im">--<br>
Hongsheng Zhao <<a moz-do-not-send="true"
href="mailto:zhaohscas@yahoo.com.cn">zhaohscas@yahoo.com.cn</a>><br>
School of Physics and Electrical Information Science,<br>
Ningxia University, Yinchuan 750021, China<br>
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-- <br>
<div dir="ltr">Amin Torabi<br>
Ph.D. Student<br>
Chemistry Department<br>
<i>The</i> University <i>of</i> Western Ontario<br>
London, On Canada, N6A 5B7<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>
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