Electronic energy levels of weakly coupled nanostructures:: C60-metal interfaces

A new approach based on density functional theory and the Anderson impurity model is developed to calculate charging energies and quasiparticle energy gaps of molecular systems weakly coupled to an environment. The approach is applied to C-60 adsorbed on Au(111) and Ag(100) surfaces, resulting in electronic structures that are in excellent agreement with recent experiments. Image-charge screening effects on molecular orbital energies are found to be of similar magnitude for the two surfaces, but charge-transfer screening and spin fluctuations also affect the Ag case due to a partially occupied C-60 orbital.