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

[S. Heidrich]

I see, thank you very much!

Kind regards
Shahri

On Tuesday, November 6, 2018 at 11:53:11 AM UTC+1, S. Heidrich wrote:
>
> 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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