Dear VASP forum,

I have a couple of questions about correcting the energy of a surface or slab model with a net dipole moment along the surface normal direction.

When performing slab calculations, a correction to the energy of a slab model with a net dipole moment along the surface normal is needed. In VASP, this correction is done using the VASP keyword IDIPOL = 1, 2, or 3. For example, if the surface normal is parallel to the c lattice vector, then the correction is done using IDIPOL = 3. A correction to the potential is also needed to obtain accurate forces using the keyword LDIPOL = .True. However, I would only like to ask about the correction to the energy.

My understanding is that the dipole moment of a periodic system is dependent on the choice of origin. If I am correct, then the size of the correction to the energy is also dependent on the choice of origin. My questions are:

1) What is the theoretical justification or basis for this correction if the correction is dependent on the choice of origin?

2) As discussed in the VASP manual or wiki page under the DIPOL keyword, to make the correction to the energy, the origin is chosen to include the plane of maximum charge density parallel to the surface, unless it is explicitly set using the DIPOL keyword. Why is this choice of origin made? Is it based on some convention?

Dipole corrections are discussed in a VASP manual page (http://cms.mpi.univie.ac.at/vasp/vasp/M ... SILON.html) and wiki page (http://cms.mpi.univie.ac.at/wiki/index. ... orrections). However, I was not able to find the answers to my questions there, in the references cited at those pages, or in previous posts at the VASP forum.

Any comments would be appreciated.

Thanks,

Yves