van der Waals Corrected Density Functional Calculations of the Adsorption of Benzene on the Cu (111) Surface

We investigate the performance of several van der Waals (vdW) functionals at calculating the interactions between benzene and the copper (111) surface, using the local orbital approach in the SIESTA code. We demonstrate the importance of using surface optimized basis sets to calculate properties of pure surfaces, including surface energies and the work function. We quantify the errors created using (3 x 3) supercells to study adsorbate interactions using much larger supercells, and show non-negligible errors in the binding energies and separation distances. We examine the eight high-symmetry orientations of benzene on the Cu (111) surface, reporting the binding energies, separation distance, and change in work function. The optimized vdW-DF(optB88-vdW) functional provides superior results to the vdW-DF(revPBE) and vdW-DF2(rPW86) functionals, and closely matches the experimental and experimentally deduced values. This work demonstrates that local orbital methods using appropriate basis sets combined with a vdW functional can model adsorption between metal surfaces and organic molecules.