Density functional theory investigation of CN on Cu(111), Ni(111) and Ni(100)

The adsorption of CN on Cu(111), Ni(111) and Ni(100) has been investigated using density functional theory (DFT). While experimental studies of CN on Cu(111) show the molecular axis to be essentially parallel to the surface, the normally-preferred DFT approach using the generalised gradient approximation (GGA) yields a lowest energy configuration with the C–N axis perpendicular to the surface, although calculations using the local density approximation (LDA) do indicate that the experimental geometry is energetically favoured. The same conclusions are found for CN on Ni(111); on both surfaces bonding through the N atom is always unfavourable, in contrast to some earlier published results of ab initio calculations for Ni(111)/CN and Ni(100)/CN. The different predictions of the GGA and LDA approaches may lie in subtly different relative energies of the CN 5σ and 1π orbitals, a situation somewhat similar to that for CO adsorbed on Pt(111) which has proved challenging for DFT calculations. On Ni(100) GGA calculations favour a lying-down species in a hollow site in a geometry rather similar to that found experimentally and in GGA calculations for CN on Ni(110).