The adsorption and reaction of O + CN → OCN on Cu(100) are studied by using density functional theory and cluster model.Cu14 cluster model is used to simulate the surface.The calculated results show that the OCN species with the molecule perpendicular to the surface via N atom (N-down) is more favorable than other adsorption models, and the N-down at the bridge site is the most favorable.For N-down, calculated OCN symmetric and asymmetric stretching frequencies are all blue-shifted compared with the calculated values of free and in good agreement with the experiments.The charge transfer from the surface to the OCN species leads to that the bonding of OCN to the metal surface is largely ionic.The present studies also show that CN with the molecule perpendicular to the surface via C atom (NC-down) at the top site is the most stable.Except NC-down at the top site, the calculated CN stretching frequencies are all red-shifted.With O coadsorbed at the hollow site, the adsorption of NC-down at the next nearest bridge or top site is energetically more favorable than that at the adjacent hollow site.The reaction of O + CN → OCN on Cu(100) has no energy barrier via both Eley-Rideal and Langmuir-Hinshelwood processes.