[CP2K-user] [CP2K:22437] Surface Dipole Moment of slab + molecule system

[Wim Ruhwinkel]

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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