<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div><br><br>符徳勝</div><div><br>2015/09/09 1:39、nilofar hadaeghi <<a href="mailto:n.hadaeghi@gmail.com">n.hadaeghi@gmail.com</a>> のメッセージ:<br><br></div><blockquote type="cite"><div><div dir="ltr"><div> I really appreciate your help. </div><div>I4/mcm is centrosymmetric. I studied the tutorial related to GaN and it helped me very much. So I conclude to calculate the difference in polarization, two structures that at least one of them should not be centrosymmetric are needed, Is it correct? </div><div>Would you please guide me whether it is allowed to calculate the difference in polarization between a strained and unstrained state of a system by implementing the berryPI on each of them separately and then subtract them ?! if it going to, what does the derived polarization describe?! Piezoelectricity(since it is the difference in polarization caused by strain) or spontaneous polarization?</div><div>Best regards,</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Sep 8, 2015 at 5:07 AM, Oleg Rubel <span dir="ltr"><<a href="mailto:orubel@lakeheadu.ca" target="_blank">orubel@lakeheadu.ca</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">That seems to be the case (<a href="http://www.cryst.ehu.es/cgi-bin/cryst/programs/nph-wp-list?gnum=140" target="_blank" rel="noreferrer">http://www.cryst.ehu.es/cgi-bin/cryst/programs/nph-wp-list?gnum=140</a>).<br>
If both structures possess the inversion symmetry, it will be pointless to calculate polarization.<br>
<span class="HOEnZb"><font color="#888888"><br>
Oleg<br>
</font></span><div class="HOEnZb"><div class="h5"><br>
> On Sep 8, 2015, at 03:48, Fecher, Gerhard <<a href="mailto:fecher@uni-mainz.de">fecher@uni-mainz.de</a>> wrote:<br>
><br>
> I thought that I4/mcm is also a centrosymmetric space group ?<br>
><br>
> Ciao<br>
> Gerhard<br>
><br>
> DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:<br>
> "I think the problem, to be quite honest with you,<br>
> is that you have never actually known what the question is."<br>
><br>
> ====================================<br>
> Dr. Gerhard H. Fecher<br>
> Institut of Inorganic and Analytical Chemistry<br>
> Johannes Gutenberg - University<br>
> 55099 Mainz<br>
> and<br>
> Max Planck Institute for Chemical Physics of Solids<br>
> 01187 Dresden<br>
> ________________________________________<br>
> Von: <a href="mailto:wien-bounces@zeus.theochem.tuwien.ac.at">wien-bounces@zeus.theochem.tuwien.ac.at</a> [<a href="mailto:wien-bounces@zeus.theochem.tuwien.ac.at">wien-bounces@zeus.theochem.tuwien.ac.at</a>] im Auftrag von Oleg Rubel [<a href="mailto:orubel@lakeheadu.ca">orubel@lakeheadu.ca</a>]<br>
> Gesendet: Dienstag, 8. September 2015 01:03<br>
> An: A Mailing list for WIEN2k users<br>
> Betreff: Re: [Wien] berryPI<br>
><br>
> The essence of electronic polarization is that it is a “relative” quantity, i.e., its calculation (and measurement too) requires a reference point. The reference structure is not always centrosymmetric. An example of non-centrosymmetric reference structure is zinc-blende GaN that is taken as P=0 in computing the spontaneous polarization of the wurtzite GaN (see <a href="https://github.com/spichardo/BerryPI/wiki/Tutorial-4:-Polarization-in-GaN" target="_blank" rel="noreferrer">https://github.com/spichardo/BerryPI/wiki/Tutorial-4:-Polarization-in-GaN</a>).<br>
><br>
> For the second part of the question on EuTiO3, enclosed structure files would certainly help. One possible strategy would be to explore a gradual evolution of the polarization between cubic and tetragonal structures.<br>
><br>
><br>
> I hope this will help<br>
> Oleg<br>
><br>
><br>
><br>
>> On Sep 7, 2015, at 3:42 AM, nilofar hadaeghi <<a href="mailto:n.hadaeghi@gmail.com">n.hadaeghi@gmail.com</a>> wrote:<br>
>><br>
>> Dear all<br>
>> I have studied some papers whose authors are S.J.Ahmed or O.Rubel in which building the centrosymmetric and noncentrosymmetric structure for the calculation of spontaneous polarization has been explained, but it is still ambiguous for me.<br>
>> Could you please help me whether the noncentrosymmetric structure must exist in the nature or it is enough to apply a displacement to the central atom of the centrosymmetric structure and then use the new structure as the noncentrosymmetric one?!<br>
>> For instance I am working on EuTiO3 in which a structural transition at temperature 282(k) from cubic to tetragonal ( from Pm3m space group to I4/mcm) occurs. Now, would you please help me whether I use the structure in I4/mcm space group as the noncentrosymmetric structure or I must change the positions of the centrosymmetric structure and use the achieved structure as the noncentrosymmetric one?<br>
>> I am using WIEN2k 14.2, Python version: 2.7.3 and Numpy version: 1.6.2.<br>
>> Sincerely yours,<br>
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