The adsorption and decomposition of C 2 N 2 on the Si(100)-2×1 surface have been calculated by the hybrid density functional B3L YP method with Si 9 H 12 and Si 15 H 16 as single- and double-dimer models, respectively. The result of our single-dimer surface model calculation shows that the surface reaction started from a single N atom of C 2 N 2 molecularly adsorbed on one silicon atom of the dimer. From the result of bond distance changes and vibrational frequency analysis, this process occurs by direct interaction of the CΞN group with a silicon dangling bond. Two different reaction paths follow; the first path occurs by the adjoined carbon atom adsorption producing four-member-ring product -Si-N-C-(CN)Si-, similar to the HCN molecule adsorbing sideway on this surface. The other path occurs by the adsorption of the second nitrogen atom with another Si atom producing a six-member-ring product, -Si-N-C-C-N-Si-. The mechanisms for the decomposition of these adsorbates have been elucidated. The result of our calculation with the double-dimer surface model reveals that the reaction barriers are somewhat lower than single-dimer system for either CC or CN bond-breaking processes but with a similar trend. The predicted stabilities of various surface species from physisorbed C 2 N 2 to chemisorbed CN radicals and silicon nitride and silicon carbide are consistent with the results of our previous studies using HREELS, XPS, UPS, and TPD methods as well as with others' data for similar reactions of CN-containing species with the Si(100)-2×1 surface. The predicted result for adsorption of two C 2 N 2 based on both surface models indicates a significant decrease in the adsorption energy for the second C 2 N 2 molecule.