A Density Functional Theory Study of Cytosine on Au(111)

The adsorption of cytosine on Au(111) is investigated using density functional theory with the nonlocal van der Waals density functional. Test calculations performed on the benzene stacked dimer and on a benzene molecule adsorbed on Au(111) allow us to assess the methodology and reveal the accuracy and predictivity of the van der Waals density funcional relative to experimental outcome. Our results for cytosine on Au(111) indicate that the inclusion of dispersion interactions is crucial for the treatment of this system. In fact, such terms enhance the value of the adsorption energy and also affect the cytosine bonding geometry: in particular, we find that a tilted geometry is always favorable relative to a parallel geometry, which was not found in standard density functional theory investigations. The combined new data for energetics and geometry lead to conclusions that contrast the common opinion that the surface-molecule interaction is negligible in the process of monolayer formation.