The optimizations used a plane wave cutoff energy of 600 Ry and Gaussian double-zeta basis sets (DZVP-MOLOPT-SR) from the work of VandeVondele and Hutter. The BFGS optimizations were considered converged when the maximal residual force was below 5 Ha bohr -1 and, simultaneously, the maximal step size (displacement between optimization steps) was below 2•10 -5 bohr. As double-zeta basis sets deliver systematically too negative adsorption energies, single-point calculations employing triple-zeta (TZVP-MOLOPT) basis sets were used to compute the energies for the optimized structures. These single-point calculations used a cutoff energy of 900 Ry.