We have examined the effect of spin contamination in single determinant wave function at the Hartree-Fock and density functional theory in studying adsorption on graphite. Particularly, we examined NO adsorption on a single-layer graphene where some carbon atoms were left unsaturated to simulate the active sites in char. Upon comparison with the results from the restricted open-shell Hartree-Fock (ROHF) method, we found that UHF has a large spin contamination. Consequently, this yields large errors in the Hartree-Fock ground-state wave function, energy-level splitting, and adsorption energy. Interestingly, the spin contamination using unrestricted density functional theory was small, and the energetic information was almost unaffected.