[CP2K-user] Electric dipole calculated using traditional operator depends on reference point for neutral periodic systems

[S. Heidrich]

Hello,

I would like to calculate the dipole of a slab with periodic boundary 
conditions along the surface plane and no periodicity perpendicular to the 
surface (so obviously only the dipole component in the latter direction is 
well-defined). From the discussion in 
https://groups.google.com/forum/#!topic/cp2k/SXhr0b6QGQk I gather that this 
should be possible in CP2K using the wavelet poisson solver and 
"traditional" dipole operator if one is only interested in the component in 
the aperiodic direction (as I am), or alternatively using the Berry phase 
method. The latter is not an option for me, as my system absolutely 
requires smearing to get a converged ground state.

To test whether this works in principle, I calculated the dipole of a small 
molecule with known dipole value (ozone, ~0.53 Debye), "pretending" it's a 
slab (XZ periodicity) and putting it in the center of the box. This seems 
to more or less reproduce the correct dipole value just fine as long as the 
reference point is near the molecule (or, equivalently, near the center), 
but if one sets the reference point near the corners of the box, there are 
huge "jumps" in the calculated dipole to unrealistic values:

[image: dipole_reference_point_dependence.png]

I'm just wondering if this is a bug or if I am doing something wrong, since 
as far as I know, the dipole calculated using the "traditional" operator 
should never depend on the reference point for a neutral system. In this 
case, it's obvious what the correct value is and that the other ones should 
just be disregarded, but if I want to apply this as a general method, it's 
a bit worrying that it can be so totally off depending on something that 
shouldn't matter. It's also not clear where the reference point should be 
in order to produce "good" results (always just the center of the box?).

As an aside, this also happens with e.g. XYZ periodicity instead of XZ and 
any Poisson solver I tried. Turning off periodicity altogether, on the 
other hand, fixes the issue, but of course that's not an option once I move 
on to actual slabs.

A minimal example input file is attached, one just needs to adjust the 
basis set and potential paths and then vary the reference point to 
reproduce this. I'm running CP2K version 6.1.

Thank you in advance for any help and suggestions!

Kind regards
Shahri
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