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 https://github.com/spichardo/BerryPI/wiki/Tutorial-4:-Polarization-in-GaN).
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. I hope this will help Oleg > On Sep 7, 2015, at 3:42 AM, nilofar hadaeghi <n.hadae...@gmail.com> wrote: > > Dear all > 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. > 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?! > 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? > I am using WIEN2k 14.2, Python version: 2.7.3 and Numpy version: 1.6.2. > Sincerely yours, > _______________________________________________ > Wien mailing list > Wien@zeus.theochem.tuwien.ac.at > http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien > SEARCH the MAILING-LIST at: > http://firstname.lastname@example.org/index.html _______________________________________________ Wien mailing list Wien@zeus.theochem.tuwien.ac.at http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien SEARCH the MAILING-LIST at: http://email@example.com/index.html