..spontaneous polarization...

[brocks]

Dear All,

i`m calculating the spontaneous polarization by berry-phase method.
i start for a centrosymmetric structure.
once i have relaxed the unit cell and positions of the ions, and this is giving me a ferroelectric "distorced" unit cell (A). i can calculate the polarization following the manual.
Using (A) and looking to the 3 runs performed with the flags LBERRY,IGPAR,NPPSTR,DIPOL
i can get the averaged values of <R>ev, <R>bp.
Naturally i can also get <R>ion for the structure (A) and as well <R>ion for the non ferroelectric "undistorced" structure (B).
Like in the case of NaF i can calculate the variation of <R>ion due to the distortion from the non ferrlectric to ferroelectric structure (this for example is done in the manual to calculate the Born effective charge).
Supposing <R>ev=0,<R>bp=0 in the structure B,
is the spontaneous polarization given by (<R>ev+<R>bp), calculated for structure A, or should i take into account also the variation of the ionic contribution (<R>ev+<R>bp)+(<R>ion_distorced-<R>ion_undistorced)?
thank you in advance.

Gianluca

[admin]

the ionic contribution has to be taken into account as well