[Wien] berryPI

[Oleg Rubel]

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.hadaeghi at 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,
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