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

## TL;DR

This paper introduces a simplified DFT method to calculate electron and hole attachment energies of molecules on insulating films supported by metals, revealing how the energy gap varies with film thickness and the influence of image interactions.

## Contribution

A new simplified DFT approach for studying frontier orbitals of molecules on supported insulating films, incorporating an implicit metal model and dielectric analysis.

## Key findings

- Energy gap increases with film thickness, matching experimental trends.
- Dielectric model explains the gap variation, dominated by image interactions.
- Overestimation of gap shift in infinitely thick films by the model.

## Abstract

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.