Extensive electronic structure calculations have been performed to understand the reaction mechanisms of the N(S-4, D-2) + CH3 reaction using ab initio multi-configurational methods. We have located a total of seven structures for the minimum on the seam of singlet/triplet potential energy crossing. According to our computational results, we conclude that triplet/singlet spin-forbidden processes are playing an essential role in this reaction in high contrast with previous theoretical studies. In addition, it is likely that singlet HCN + H-2 products are formed through so-called 'roaming' dynamics. (C) 2014 Elsevier B.V. All rights reserved.