We preformed hybrid, embedded cluster calculations at the DFT/MM level to investigate cyclization of C6 diene leading to C6 cyclic product in HZSM-5. We observe that C6 diene cyclization can occur either directly from a physisorbed olefin or by first chemisorption to form an alkoxide intermediate. Furthermore. chemisorption can occur through a double-site mechanism or a single-site mechanism. We also observe two alternative pathways for cyclization starting from chemisorbed alkoxides. Primary alkoxides preferred direct 1.6 ring closure (17.7 kcal/mol activation energy), whereas secondary alkoxides only proceeded through a 1.5 ring closure to methyl eye lope it ten e alkoxide (22.12 kcal/mol activation energy), followed by ring expansion through a stable bicyclo[3. 1.0]hexane intermediate (21.31 kcal/mol activation energy for the rate-limiting step). Thus our calculations confirm that C6 dienes are likely to be the precursors for cyclization reactions. Finally, we show that embedded Cluster calculations are essential for proper analysis of C6 olefin chemistry in ZSM-5 models-with fairly important distinctions between embedded Cluster and bare cluster results. (c) 2004 Published by Elsevier Inc.