  We present a simplified density functional theory (DFT) method to com- pute
vertical electron and hole attachment energies to frontier orbitals of
molecules absorbed on insulating films supported by a metal substrate. The
adsorbate and the film is treated fully within DFT, whereas the metal is
treated implicitly by a perfect conductor model. As illustrated for a pentacene
molecule adsorbed on NaCl films sup- ported by a Cu substrate, we find that the
computed energy gap between the highest and lowest occupied molecular orbitals
- HOMO and LUMO -from the vertical attach- ment energies increases with the
thickness of the insulating film, in agreement with experiments. This increase
of the gap can be rationalized in a simple dielectric model with parameters
determined from DFT calculations and is found to be dominated by the image
interaction with the metal. However, this model overestimates the down- ward
shift of the energy gap in the limit of an infinitely thick film. This work
provides a new and efficient strategy to extend the use of density functional
theory to the study of charging and discharging of large molecular absorbates
on insulating films supported by a metal substrate.
