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[yukihiro_okuno@fujifilm.co.jp]

Dear Yu Chol Jun.

For the calculation of spontaneous polarization, we must 
calculate the spontaneous polarization of the system 
by moving atom's position from the centrosymmetric position to the 
fully relaxed ferroelectric position, because the observed spontanious 
polarization is the difference between some centro-symmetric 
state and ferro electric state.
The spontaneous polarization has ambiguity of  e*c/Omega ( c is lattice 
vector 
whose derection is parallel to the polarization which we calculate,
 Omega is volume)

read, 
R.Resta   Rev Mod Phys Vol66  899 (1994) 

For example move all atom like 

x = x0 + a * (x1-x0)   (0 < a < 1)

x0  centro symmetric position
x1  ferro electric position
for all atoms with same "a",
and plot spontanious polarization for "a" and plot the 
polarization and check plotted line. If the line has 
discontinuity, plus  integer * ec/Ometa (mod part) in 
order to sets the line bocome continuity.
The difference between the centro-position spontanious polarization 
and ferroelectric position is the "observed" spontanious polarization.

Yukihiro Okuno