Polarization calculation VASP

[chandan_kumarvishwakarma1]

Dear Users
I am trying to calculate spontaneous polarization for rhombohedral structure
of sodium bismuth titanate (NBT) using VASP. For this I have used
LCALPOL = .TRUE. in the INCAR file but, I am confused about the "DIPOL" tag.
Please suggest me how can I specify DIPOL tag.
Here I am attaching POSCAR of the Polar structure:

====================
NBT
1.00000000000000
3.2054789586762999 0.0000000000000000 4.6668922818096190
-1.6027395304237013 2.7760262896936472 4.6668922819416165
-1.6027395304237013 -2.7760262896936472 4.6668922819416165
O Na Ti Bi
6 1 2 1
Direct
0.2746092000928400 0.7698783260068538 0.7698783260068538
0.7898604393586623 0.2723430799424094 0.2723430799424094
0.7698783260339894 0.2746092001739626 0.7698783259176309
0.2723430799668023 0.7898604394627711 0.2723430798397501
0.7698783260339894 0.7698783259176309 0.2746092001739626
0.2723430799668023 0.2723430798397501 0.7898604394627711
0.7432292387509349 0.7432292387553900 0.7432292387553900
0.0005668014061513 0.0005668014542266 0.0005668014542266
0.5110039860833950 0.5110039861424838 0.5110039861424838
0.2266975223064520 0.2266975223045193 0.2266975223045193
===================

---
Chandan

[henrique_miranda]

I believe that the information you are looking for is documented here.
https://www.vasp.at/wiki/index.php/DIPOL

The main point to keep in mind is that the polarization is defined up to an integer multiple of the polarization quantum.
You can read a general explanation about why it is so here:
https://arxiv.org/abs/1202.1831

The position specified in DIPOL is a way to choose the polarization branch that you get.

[chandan_kumarvishwakarma1]

Dear henrique_miranda
Thanks for your reply, but I am still confused about the DIPOL tag.
I read the documentation but that is not clear to me, It is saying
"the center of the cell w.r.t. which the total dipole-moment in the cell is calculated"
So my question is how can we decide the center.
For better explanation I had attached the POSCAR, So please explain w.r.t. my structure.

---
Chandan

[henrique_miranda]

I think the answer to your question is documented here:
https://www.vasp.at/wiki/index.php/DIPOL

In particular:

You should use this option only for surfaces and isolated molecules

If you are studying a bulk system, this tag is not relevant.
You can also find this yourself by performing a few calculations with different positions of DIPOL and see if your final result changes.