how to define the DIPOL when doing a berry phase calculation?

[zj]

Dear all,

From the manual, it said that the DIPOL is the "center of cell". Can someone give a more detailed explanations? It is the mass center or the position center, or something else?

I want to calculate the polarization of the whole cell. For example a tetragonal BaTiO3. What is the DIPOL value for the following POSCAR:

tet-BaTiO3
1.00
3.992 0.0 0.0
0.0 3.992 0.0
0.0 0.0 4.036
3 1 1
Direct
0.0000000000 0.5000000000 0.485
0.5000000000 0.0000000000 0.485
0.5000000000 0.5000000000 -0.025
0.5000000000 0.5000000000 0.5135
0.0000000000 0.0000000000 0.000


I tried set DIPOL=0.25 0.25 0.25, it seems to get a reasonable value of polarization. However, if I set other value, I can not get the same polarization.

In the example of NaF on the manual, it only moves one atom at z direction, so it can get the Born effective charge of F. However, in the above example of BaTiO3, I move 4 atoms at one time, so, can I get the correct polarization with this POSCAR. OR, I have to get all the Born effective charge of all the atoms and then calculate the polarization by hand?

Thank you for your reading.

Best wishes,

Jian

[admin]

1) DIPOL has to be defined corresponding to the center of the ionic contribution to the dipole moment in the cell.
2) concerning your second question: the example given in the VASP online manual correspond to one paradigm polarisation mode, it would be beyond the scope of the manual to deal with more complicated examples.
The dipole term evaluated from VASP (following the steps as discussed in the manual)
corresponds to the change in the dipole due to the collective pattern of
dislocations. If it has to be analysed with respect to the specific ionic contributions, the polarisation mode has to be disentangled.