Adsorption energy in electric field

[obrovko]

I am trying to calculate the adsorption energy of small molecules (H2,CO,CO2) on a sheet of graphitic boron nitride. I've taken as a reference DMol calculations, which seem to be reasonable [PNAS 107(7), 2801 (2010)].

The adsorption energies and bond lengths without the field seem to be as good as they get with GGA/LDA. Yet when I turn on the field the adsorption energies calculated as E(subst+mol,ef)-E(subst,ef)-E(mol,ef) turn out to be higher (less favorable adsorption) regardless of the field direction. The relaxations and charge redistribution according to the Bader analysis are all reasonable, but the energetics seems to be faulty. I wonder what I am doing wrong.

Main INCAR keys:

PREC = A
EDIFF = 1.0E-4
ENCUT = 400
GGA = 91 (PBE and LDA give qualitatively similar results)
VOSKOWN = 1
LREAL = .FALSE.

ISMEAR = 0
SIGMA = 0.05

EDIFFG = -0.005
IBRION = 2
ISIF = 2
POTIM = 0.1

EFIELD = 0.0
LDIPOL = T
IDIPOL = 3

Cell - orthorombic with 3x4 hexagon BN cells
Potential for H - 1 valent electron

I'd be very grateful if anybody could give me a hint as to what I am doing wrong.
Should the energy be calculated differently? Is there some sort of double-counting of the interaction with the dipole creating the field when I subtract the energy of the molecule and the BN on their own?

Best regards,
Oleg

[obrovko]

Does anybody have an idea what might be wrong?