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Dearu Vasp Users.

I'm calculatiing polarization for the ferroelectric material.
A1_{0.5}A2_{0.5}BO3, where A sites in perovskite is arranged
A1 ion and A2 ion with [001] ordering.

For getting the sponranious polarization, I compared the
polarization between distorted (ferroelectric position) and
undistorted (ideal perovskite position).
But I found the ionic term has large discontinuity at the undistorted
position,, like below.

I found that ionic term of <R>ion (Polarization) for distorter ferroelectric
potision, like

ionic term:<R>ion
<R> = ( 98.40638, 98.40638, -1.73621 ) electrons Angst

and ideal position,
ionic term: <R>ion
<R> = ( 98.40638, 98.40638, 38.60456 ) electrons Angst

This is large discontinuity -1.73621(distorted) and 38.60456 (ideal).
I had move atoms from ideal perovskite position to distorted position,
I had found the discontinuty of ionic <R> term at just the ideal perovskite
position.

I had set DIPOL=0.0,0.0,0.0 and also DIPOL=0.5 0.5 0.5.
These two case I got such a discontinuity.

The manual said that when compare the distorted and undistorted
system. we should take both system must be the same side of dipole,
but how we set DIPOL when we get the spontanious polarization
for such a A site orderd complex perovskite.


The position of my calculated system is as follow,

# for POSCAR undistorted.
1.00000000000000
3.9362552663422279 0.0000000000000000 0.0000000000000000
0.0000000000000000 3.9362552663422279 0.0000000000000000
0.0000000000000000 0.0000000000000000 8.5787917413004742
1 1 2 6
Direct
0.0 0.0 0.00
0.0 0.0 0.50
0.5 0.5 0.25
0.5 0.5 0.75
0.5 0.5 0.00
0.5 0.5 0.50
0.5 0.0 0.25
0.0 0.5 0.25
0.5 0.0 0.75
0.0 0.5 0.75

# POSCAR for distorted.
1.00000000000000
3.9362552663422279 0.0000000000000000 0.0000000000000000
0.0000000000000000 3.9362552663422279 0.0000000000000000
0.0000000000000000 0.0000000000000000 8.5787917413004742
1 1 2 6
Selective dynamics
Direct
0.0000000000000000 0.0000000000000000 0.0 F F T
0.0000000000000000 0.0000000000000000 0.580641528170233 F F T
0.5000000000000000 0.5000000000000000 0.258670031604616 F F T
0.5000000000000000 0.5000000000000000 0.731572992033364 F F T
0.5000000000000000 0.5000000000000000 0.943182345842004 F F T
0.5000000000000000 0.5000000000000000 0.469346215366166 F F T
0.5000000000000000 0.0000000000000000 0.217453532607841 F F T
0.0000000000000000 0.5000000000000000 0.217453532607841 F F T
0.5000000000000000 0.0000000000000000 0.696542229605392 F F T
0.0000000000000000 0.5000000000000000 0.696542229605392 F F T

My Input Berry Phase calculation is
SYSTEM = A1A2BO3

Startparameter for this Run:
NWRITE = 2; LPETIM=F write-flag & timer
ICHARG = 11 initial charge 0-calc 1-from CHGCAR, 2-super potision


Electronic minimization
ALGO = NORMAL # default for IALGO=38
IALGO = 38 # default Davodason block iteration scheme
EDIFF = 1E-06 # stopping -criterion for ELM
PREC = High


GGA = 91 # GGA PB PW LM 91 PE RP

ISMEAR = -5
SIGMA = 0.2

Berry Phase
LBERRY = .TRUE.
IGPAR = 3
NPPSTR = 15
DIPOL = 0.0 0.0 0.

the large contribution to the ionic dipole certainly mostly stems form the large shift of the first O atom (atom #5) from one side of the unit cell to the other (z=0.0 --> 8.088).
the center of the dipole moment of the cell can easily be calculated by hand.

Dear admin.
Thank you for your reply.

But I set first O atom (atom#5) position as

0.5 0.5 1.00
(for undistorted)
0.50 0.50 0.943182345842004 F F T
(for distorted),

and DIPOL=0.0 0.0 0.0,

I also found large shift for Ionic term,

ionic term : <R>ion
<R> = ( 98.40638, 98.40638, 38.60456 ) electrons Angstor R_<ion>

for distorted case,
ionic term: <R>ion
<R> = ( 98.40638, 98.40638, -0.49932 ) electrons Angst

I found the discontinuity just at undistorted position when I moved atom like below
z_{\lambda}=z_{0}+\lambda{z-z_{0}}
(I changed \lambda=0.0 to 1.0, z is distorted atom potision and
z_{0} for atom position. z_{0}=1.0 and z = 0.943182345842004
for forst O atom (atom #5).

like \lambda=0.1
ionic term: <R>ion
<R> = ( 98.40638, 98.40638, -11.63130 ) electrons Angst.

It seems strange.

And the center of dipole moment is given below?

r_{+} = \Sum_{i} Z_{i} *r_{i} /\Sum_{i} Z_{i}
for (plus charge ion : i run for plus ion)

and

r_{-} = \Sum_{i} Z_{i} *r_{i} /\Sum_{i} Z_{i}
for (minus charge ion : i run for minus ions r_{i} is minus ion position)

and the center of dipole r_{0} is just the center of plus ion cnter r_{+}
and minus ion center r_{-},

r_{0} = 0.5 * (r_{+} + r_{-})