On the organic energy gap problem

In conjugated organic molecules, the difference between the HOMO and LUMO Kohn–Sham eigenvalues is significantly smaller than the transport gap measured experimentally. We discuss here, within a local-orbital formulation of DFT, how this problem can be corrected using appropriate hybrid potentials, that add a fraction of Hartree–Fock exchange interaction in the DFT calculation. We illustrate this approach presenting calculations for two simple systems: H2 and C6H6; then, we discuss how to implement this hybrid approach in a general local-orbital calculation, adjusting the hybrid contribution to yield the correct experimental HOMO/LUMO energy gap for the molecule. We also consider the case of an organic molecule on a metal and analyze the effect of the molecule–metal interaction on the organic energy gap. In particular, we discuss how to introduce in this hybrid-potential scheme the effect of the image potential, and present results for the organic molecules PTCDA, TTF, benzene and pentacene on the metal surfaces Au(111), Ag(111) and Cu(111). (paper)