Density functional theory (DFT) and hybrid quantum mechanics/molecular mechanics (QM/MM) calculations were used to study stable intermediates for alkene epoxidation using Mn-porphyrin catalysts. For the reaction intermediate involving complexation of the alkene with the oxidized Mn-porphyrin, four intermediates have been proposed in the literature. A concerted mechanism with no intermediate has also been proposed, and these five mechanisms could all involve the formation of a product complex. Our calculations show that the product complex has the lowest energy, followed by the radical intermediate. The metallaoxetane intermediate is much higher in energy, and the calculations do not support carbocation or pi-radical cation intermediates. A polarizable continuum model was used to account for solvent effects, and the calculated energies of solvation are comparable for all minima, along the reaction path. (c) 2008 Elsevier B.V. All rights reserved.