dipole correction for asymmetric slab

[cmm]

Dear VASP users,

I have a question regarding dipole corrections for an asymmetric slab.

Currently I am using the flag IDIPOL=3 (vacuum along the z-axis) to apply the dipole corrections, letting VASP determine the dipole center.
The VASP manual mentions that the resultant dipole correction is scaled by the dielectric constant and that, for 3D solids one should define this in the INCAR, while for isolated atoms and molecules one can use the vacuum dielectric constant. I am however uncertain what to do in the case of slabs.

My own tests seem to suggest that my dipole corrections are wrong by what seems to be a factor 2 (though this could be coincidence). The dielectric constant should be around 8-10 for the bulk material. My question now is: am I supposed to use an epsilon for scaling and, if yes, how do I obtain it?

If not, what effect am I seeing here?

For clarification I have uploaded my calibration test results ( http://imgur.com/AN4tv ). Each datapoint represents a pseudo adsorption energy, ie. a slab with an NH3 molecule placed on top (but not relaxed) minus the energy of the empty surface and the (constant) energy of the isolated molecule. For each datapoint the vacuum for both the adsorbed and empty slab are increased.
Both the dipole-corrected and uncorrected datasets seem to converge equally slow, but their average (which is equal to what an epsilon=2 would give) converges much faster, with the first datapoint already differing by less than 0.01 eV from the last.

Thanks,
Stefaan

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