[Pw_forum] polarization calculation in ferroelectrics

[hania djani-ait aissa]

Dear all, 
I am new PWSCF user, I am trying to calculate polarization in Bi4Ti3O12. this  material is centered tetragonal in its paraelectric phase and orthorhombic bases centered in its ferroelectric one. having in mind the papers of Resta, King-smith and Vanderbilt, i have to fix the initial state with P=0 and then make a small displacement of a subgroup with respect to others. my question is: what will be  the initial structure to put as input?  the paraelectric tetragonal one with its symmetry center, or the ferroelectric one were i force the orthorhombic structure to be centered , relaxing all atoms and making the small displacement??
thank you in advance,
Hania

H. Djani Ait Aissa
Center of Development of Advanced Technologies
Haouch Oukil, PoBox.017, Algiers, Algeria
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