If you check Figure 2 of PhysRevLett.109.267602, you will find that there is NO 
polarization in I4/mcm !
and in the  PhysRevLett.97.267602 you find "... the crystallographic  symmetry  
becomes P4/mm", now compare this to the Fig.2 of the first reference, what do 
you see ?


Ciao
Gerhard

DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:
"I think the problem, to be quite honest with you,
is that you have never actually known what the question is."

====================================
Dr. Gerhard H. Fecher
Institut of Inorganic and Analytical Chemistry
Johannes Gutenberg - University
55099 Mainz
and
Max Planck Institute for Chemical Physics of Solids
01187 Dresden
________________________________________
Von: wien-boun...@zeus.theochem.tuwien.ac.at 
[wien-boun...@zeus.theochem.tuwien.ac.at] im Auftrag von Sheikh Jamil Ahmed 
[sahm...@lakeheadu.ca]
Gesendet: Dienstag, 8. September 2015 23:06
An: A Mailing list for WIEN2k users
Betreff: Re: [Wien] berryPI

I would like to add that EuTiO3 seems to be a multiferroic compound with ground 
state being antiferromagnetic- paraelectric system (I4/mcm). Then, under some 
compressive or tensile epitaxial strain, several types of transition is 
possible (For example, antiferromagnetic- ferroelectric or ferromagnetic- 
ferroelectric) depending on the magnitude and type of strain. The following 
articles address this issue very well.
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.109.267602

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.97.267602


Hope that helps.

Sheikh

--
Sheikh Jamil Ahmed
PhD Student
Department of Material Science and Engineering
McMaster University
1280 Main Street West
Hamilton, Ontario, Canada L8S 4L7


On Tue, Sep 8, 2015 at 3:07 PM, Oleg Rubel 
<oru...@lakeheadu.ca<mailto:oru...@lakeheadu.ca>> wrote:
To address your questions...

> 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?

That’s correct. When the structure file contains the inversion symmetry, Wien2k 
switches from complex to real version for wave functions. Then there is no 
phase information to be processed.

> 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?

You are heading towards piezoelectric coefficients. There are some 
technicalities, such as clamped vs relaxed ion approximation, proper vs 
improper. Here is a good reference: http://arxiv.org/pdf/cond-mat/9903137.pdf
I do not see any barriers from BerryPI perspective.


Best regards
Oleg

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