The equilibrium structures, vibrational properties, and interaction energies for four bimolecular complexes of HCN and HNC (HCN...HCN, HNC...HNC, HCN...HNC, and HNC...HCN) were studied with ab initio methods. Different electron correlation levels and basis sets up to CCSD(T)/6-311++G(2d,2p) were used in the geometry optimizations and interaction energy calculations, and the vibrational frequencies were calculated at the MP2 and MP3 levels of theory. To study the nature of the intermolecular interactions, an energy decomposition analysis was carried out. For the HNC...HCN complex, which possesses a hydrogen bridge between two carbons, the counterpoise-corrected interaction energy and its decomposition has been calculated as a function of the intermolecular distance. The effect of subunit deformation on the interaction energy has also been considered, The results indicate that the HNC dimer has considerably stronger intermolecular interaction than the HCN dimer, and the HNC...HCN complex is bonded as strongly as the HCN dimer. The comparison of the various properties of the complexes and the energy decomposition analysis in particular indicate that carbon can act as both a hydrogen acceptor and donor, and the resulting hydrogen bonds in these complexes are relatively strong.