In all the employed slabs, the reciprocal space was sampled using optimized Monkhorst-Pack k-point meshes of 3×3×1 dimensions. The geometric optimizations were carried out using a plane-wave basis set with a kinetic energy cutoff of 415 eV, enough to reach the chemical accuracy of ~0.04 eV as tested on adsorption energies, see below. The electron density optimization criterion was set to 10 -6 eV, while atomic relaxation stopped when consecutive geometries yielded differences in energy below 10 -5 eV. All calculations were considered closed shell, i.e. non-spin polarized, except for Co, Ni, and Fe magnetic surfaces . The placed CO molecule was optimized likewise isolated in the centre of a 10×10×10 Å 3 cubic unit cell and optimized at Γ-point. For evaluating the energy of isolated C and O atoms, the procedure was similar to the CO molecule but placed inside a 9×10×11 Å 3 broken-symmetry unit cells, so as to force and gain their correct orbital occupancies.