The adsorption of cyanide (CN) and the coadsorption of CN+O on Ni(100) are treated using an ab initio embedding theory. The Ni(100) surface is modeled as a three-layer, 30-atom cluster with the Ni atoms fixed at bull; lattice sites. The present calculations show that CN is able to bind to the surface either via the carbon, or nitrogen, or in a side-on geometry with very small differences in total energy, less than or equal to 2 kcal/mol. Adsorption energies at fourfold, bridge and atop sites are comparable. For N-bonded CN, the adsorption energy is 114 kcal/mol at the most favorable (fourfold) site. For C-bonded CN, the adsorption energy is 115 kcal/mol at the most favorable (atop) site. For the side-on bonded CN, the calculated adsorption energy is 113 kcal/mol. Although CN is strongly bound to the surface, the molecule is nearly free to rotate to other geometries over very small energy barriers, less than or equal to 2 kcal/mol. Dipole moment calculations show that the bonding of CN to the Ni surface is largely ionic. On coadsorption of CN+O on Ni(100), the adsorbed atomic oxygen tends to block CN adsorption at the nearby surface sites. With O coadsorbed at a fourfold site, the adsorption of CN at the next nearest bridge or on-top sites are energetically more favorable than adsorption at the adjacent fourfold site, and the C-N bond is perpendicular to the surface. It is predicted that the reaction of O+CN-->OCN is exothermic and the formation of surface OCN is due to the formation of C-O bond by the interaction between the adsorbed O and the C atom of adsorbed CN. (C) 1997 American Institute of Physics. [S0021-9606(97)00944-6].