The adsorption, isomerization, and decomposition of HCN on Si(100)-2 × 1 surface have been investigated by means of a density functional theory calculation using a double-dimer cluster model. The results revealed that both HCN and its HNC isomer can be readily adsorbed on a Si-Si dimer either dissociatively or molecularly in an end-on and a side-on configuration. Side-on adsorption occurs by the cycloaddition of the C≡N group on to the Si-Si dimer, whereas dissociative adsorption gives rise to H(a) and CN(a) adspecies initially via the end-on configuration on the same dimer or across the two dimers. Adsorbate-adsorbate interactions and reactions have also been studied with two HCN molecules. For the end-on adsorption, the first HCN(a) exerts a significant effect on the adsorption geometry of the second HCN. In particular, a synergetic effect has been observed for the parallel adsorption of two HCNs with their C=N groups bridging across the two dimers. For the side-on adsorption, the adsorbate-adsorbate interaction is negligible with minor effects on the adsorption geometry. H-migration between the two neighboring, side-on HCN admolecules can occur readily, leading to the formation of HCNH(a) and NC(a) surface species. The calculated vibrational frequencies of the HCNH(a) and NC(a) adspecies are in good agreement with the experimental HREELS data of the HCN/Si(100) system.