Frontier molecular orbitals of single molecules adsorbed on thin insulating films supported by a metal substrate: A simplified density functional theory approach

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.