and a plane wave cutoff of 600 Ry. A maximal residual force of 5•10 -6 Ha bohr -1 and a maximal displacement of 2•10 -5 bohr between steps were used as convergence criteria. After successful optimizations, total energies were recomputed using larger triple-zeta basis sets (TZVP-MOLOPT) and a plane wave cutoff of 900 Ry. For a given configuration, the adsorption energy was then calculated as: