<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 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 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 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 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 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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</div></div></blockquote></div><br><br clear="all"><br>-- <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><span style="font-family: 'Droid Sans',arial,sans-serif; font-size: 13px; border-collapse: collapse; color: rgb(32, 32, 32);">Phone: <a value="+15196612111" style="color: rgb(103, 117, 58);">519-661-2111</a> Ext: 87871</span><div>
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