A cluster DFT (B3LYP) calculation study of CIN3 and BrN3 dissociation pathways catalyzed by molecular sieve is reported. A three-tetrahedral molecular sieve cluster (Al(OHSiH3)(OSiH3)(OH)(2), called T3) has been chosen in this study. Triplet-state products CIN and BrN are more easily produced with T3 molecular sieve cluster. Although the potential energy surfaces with T3 molecular sieve cluster are similar with those without T3 molecular sieve cluster, the dissociation process of CIN3 and BrN3 with T3 molecular sieve cluster becomes easier, which can be attributed to smaller energy gaps from the ground state reactant to the intersystem crossing point. (C) 2004 Elsevier B.V. All rights reserved.