Multiconfigurational self-consistent field calculations have been performed to investigate structural and electronic properties of cubic siliconcarbide (001) (SiC (001)) surfaces. The dimer on silicon-terminated SiC (001) (Si–SiC (001)) is found to be diradical in nature, due to destabilization of the π bond by bending the dimer. Since the SiClattice constant is larger than that of diamond, the >C=C< dimer on the carbon-terminated SiC (001) (C–SiC (001)) surface is flatter and its π bond is stronger than those on diamond (001). The bridging dimer on the C–SiC (001) exhibits relatively small multiconfigurational character despite its bent geometry. H2adsorption onto the Si–SiC (001) diradical dimer is more favorable than that onto the partial π bonded Si (001) dimer. As the dimer geometry becomes flatter, the π bond becomes stronger and the H2adsorption on the dimer becomes less favorable.