  The theoretical modelling of metal-organic interfaces represents a formidable
challenge, especially in consideration of the delicate balance of various
interaction mechanisms and the large size of involved molecular species. In the
present study, the energies of interface states, which are known to display a
high sensitivity to the adsorption geometry and electronic structure of the
deposited molecular species, have been used to test the suitability and
reliability of current theoretical approaches. Two well-ordered overlayer
structures (relaxed and compressed monolayer) of NTCDA on Ag(111) have been
investigated using two-photon-photoemission to derive precise interface state
energies for these closely related systems. The experimental values are
reproduced by our DFT calculations using different treatments of dispersion
interactions (optB88, PBE-D3) and basis set approaches (localized numerical
atomic orbitals, plane waves) with remarkable accuracy. This underlines the
trustworthiness regarding the description of geometric and electronic
properties.
