Abstract Total energy calculations in the generalised gradient approximation of density functional theory, using the castep computer code, have been used to determine the equilibrium geometries of NH 3 and CH 3 on Cu(1 1 1). Consistent with the results of both previous calculations and scanned-energy mode photoelectron diffraction (PhD) experiments, ammonia is found to adsorb in atop sites with an adsorption energy of approximately 0.7 eV. The Cu–N nearest-neighbour bondlength shows some coverage dependence, the best agreement with experiment being achieved at lower coverage. The adsorbed methyl species, however, shows the highest adsorption energy for a threefold coordinated hollow site, as found in a PhD experimental study, with a strong preference for the C–H bonds to point towards the nearest-neighbour Cu atoms. The results, which imply involvement of both the C and the H atoms in the substrate bonding, are consistent with previous theoretical studies of methyl adsorption on Ru(0 0 0 1) and Ni(1 1 1), but quite unlike those of similar studies on Pt(1 1 1) and other metal surfaces.