Ab initio and nonlocal density functional theory (DFT) calculations were performed to determine reaction mechanisms for formation of the six-membered ring C3N3Cl3 (cyanuric chloride) from the monomer cyanogen chloride (ClCN). MP2 geometry optimizations followed by QCISD(T) energy refinements and corrections for zero-point energies for critical points on the potential energy surface were calculated using the 6-31G* and 6-311+G* basis sets. DFT (B3LYP) geometry optimizations and zero-point corrections for critical points on the potential energy surface were calculated with the 6-31G*, 6-311+G*, and cc-pVTZ basis sets. Good agreement is found for MP2 and DFT geometries and frequencies of cyanuric chloride and ClCN when compared with experimental values. Two formation mechanisms of cyanuric chloride were investigated, the concerted triple association (3 ClCN --> cyanuric chloride) and the stepwise association (3ClCN --> Cl2C2N2 + ClCN --> cyanuric chloride). All calculations show that the lower energy path to formation of cyanuric chloride is the concerted triple-association. MP2 and DFT intrinsic reaction coordinate calculations starting from the transition state for concerted triple association reaction proceeding toward the isolated monomer resulted in the location of a local minimum, stable by as much as 8.0 kcal/mol, that corresponds to a weakly-bound cyclic (ClCN)(3) cluster. The existence of this cluster on the reaction path for the concerted triple association could lower the entropic hindrance to this unusual association reaction mechanism. The DFT/cc-pVTZ barrier to concerted triple association relative to isolated ClCN is 42.9 kcal/mol. The QCISD(T)//MP2/6-311+G* barrier to concerted triple association is 41.0 kcal/mol. The DFT/cc-pVTZ barrier to formation of the dimer (stepwise association reaction) is 63.4 kcal/mol while the QCISD(T)//MP2/6-311+G* barrier is 76.7 kcal/mol. The barrier to formation of cyanuric chloride relative to the (ClCN)(3) minimum requires similar to 46-49 kcal/mol, indicating that the concerted triple-association reaction via formation of the (ClCN)(3) prereaction intermediate is the lower energy path to formation of cyanuric chloride. The temperature-corrected (T = 298 K) heats of reaction for formation of cyanuric chloride from ClCN are -63.4 and -61.2 kcal/mol for the B3LYP/cc-pVTZ and the QCISD(T)//MP2/6-311+G* predictions, respectively.