Van der Waals effects at molecule-metal interfaces

We present the results of plane-wave pseudopotential periodic density functional theory (DFT) calculations on the geometries, energetics, and electronic structure of small molecules on Au(111). The chosen molecules - benzene, ammonia and cytosine - are representative of different adsorption regimes and interaction strengths. The chosen substrate is a prototype noble-metal surface that is widely employed as a support for organic materials. We assess the relevance of van der Waals effects in the adsorption process and the accuracy of different first-principle density functionals that have been recently developed to embody such effects. We find that there is no unique functional that is optimal for any system. In particular, our results reveal that functionals designed to reduce the short-term repulsion between the adsorbate and the substrate usually overestimate the adsorption strength and may even predict the wrong adsorption orientation. We show that an accurate description of the substrate does not ensure an accurate evaluation of the adsorption energetics, while the electronic structure is less sensitive to the specific choice. We propose the best choice for DFT calculations of DNA bases on Au(111) and similar systems in which both short-range and long-range interactions exist.