[CP2K-user] [CP2K:22440] Surface Dipole Moment of slab + molecule system

[Thomas Kühne]

Dear Wim,

I found an inconsistency in the traditional moment calculation and submitted a fix in PR #6274 (https://github.com/cp2k/cp2k/pull/6274). 
Periodic image contributions included in the overlap matrix were missing from the dipole operator, which explains how the reported total 
charge could be correct while the dipole depended on the reference point.

In my small partially periodic test, the corrected slab-normal dipole is independent of the reference point and consistent with density 
integration within the grid accuracy. However, I cannot reproduce your exact numbers without the coordinates referenced by Au_NHC.xyz, 
which were not included in the attachments.

For your XY-periodic slab, the z component is well-defined with MOMENTS PERIODIC F, including with smearing. The in-plane components 
are not uniquely defined, so the reported “Total” magnitude should NOT be interpreted as the slab dipole!

Cheers,
Thomas

> Am 13.10.2026 um 17:16 schrieb Wim Ruhwinkel <wruhwinkel at gmail.com>:
> 
> Dear CP2K community,
> 
> I am trying to compute the surface-normal dipole moment of an NHC-type molecule adsorbed on an Ag(111) slab (258 atoms; hexagonal cell, a = b = 23.068 Å, γ = 120°, c = 50 or 80 Å). The system is periodic in XY (set in &CELL and &POISSON) and charge neutral. Settings: CP2K 9.1, GPW, optPBE-vdW, Fermi-Dirac smearing at 300 K, POISSON_SOLVER ANALYTIC. The input is attached.
> 
> Since the Berry-phase formula does not support smearing, I use &MOMENTS with PERIODIC .FALSE. The output reports Electronic = −2272, Core = 2272, Total = 0.000, but the z-dipole depends strongly on the reference point:
> 
> | REFERENCE    | c = 50 Å | c = 80 Å |
> |--------------|----------|----------|
> | COM          | 106.1 D  | 105.1 D  |
> | COAC         | 111.1 D  | 110.1 D  |
> | USER_DEFINED | 775.0 D  | 2100.6 D |
> | ZERO         | 2503.5 D | 4160.7 D |
> 
> For a neutral system the dipole should be independent of the reference point. The same input with a single H2O molecule in the same 2D cell gives the correct 1.806 D (non-periodic reference: 1.808 D), so the setup itself seems fine to me.
> 
> The physically expected value is about 10–13 D: integrating the plane-averaged TOT_DENSITY_CUBE along z gives 10.4–12.8 D. The value printed by SURFACE_DIPOLE_CORRECTION in fully periodic runs is −2.16 e·Å (10.4 D). With WAVELET (CP2K 2026.2) &MOMENTS gives 96 D. TOTAL_DIPOLE in &LOCALIZE also gives unphysical values.
> 
> My questions:
> 1. Is the traditional dipole operator in &MOMENTS (PERIODIC .FALSE.) applicable to a system that is periodic in x/y, at least for the non-periodic z component? If not, why does it depend on the reference point?
> 2. Is there a recommended way to obtain the slab dipole directly from CP2K without applying SURFACE_DIPOLE_CORRECTION?
> 
> Thank you very much for your time.
> 
> Best regards,
> Wim Ruhwinkel
> 
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> <Ag_NHTe_energy.inp><QS_optPBE-vdW.inc><basis_pseudo.inc>

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