...electronic polarization with Berry-phase....

[brocks]

Dear All,

i have some doubts about how to calculate the polarization with Berry-phase theory in VASP.
I tried to reproduce the theoretical value of 88uC/cm**2 in the case of PbTiO3.
This value is coming from PRL 95, 257601 (2005).
Simply i did the following:
-i relaxed the unit cell (ISIF=3) of PbTiO3.
i got the structural parameters of table II of the PRL paper.
-then i followed the procedure described in the manual of VASP to calculate the polarization.
The first run produce the CHGCAR and the second one is the Berry phase (LBERRY =.TRUE.) calculation (i just did it along c (IGPAR = 3) because this is the direction of the polarization).
By the command:
grep "<R>" OUTCAR
i can get:
Expectation value term: <R>ev
<R> = ( 0.00000, 0.00000, x ) electrons Angst
Berry-Phase term: <R>bp
<R> = ( 0.00000, 0.00000, y ) electrons Angst
ionic term: <R>ion
<R> = ( xx, yy, zz ) electrons Angst

My guess is that to get the 88uC/cm**2 i have to divide (x+y) for the volume of the unit cell and then to change from units electrons*Angst to uC/cm**2.
Actually in this way i get a value very much close to their 88uC/cm**2 but i would like to be sure about the procedure i`m using to get it.
thank you.

Gianluca

[support_vasp]

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