In this paper we have used a DFT (B3LYP) approach to investigate the potential energy surface for the reaction between ethylene and (chloromethyl)zinc chloride (ClCH2ZnCl), which represent a model system for the Simmons-Smith cyclopropanation reaction. Two reaction channels have been found: one leads to the cyclopropane product (addition channel) and the other to the propene product (insertion channel). The addition reaction has an activation energy of 24.7 kcal mol(-1) and, as experimentally found, is favored with respect to the insertion, which is characterized by a larger activation energy (36.0 kcal mol(-1)). The addition transition state corresponds to a three-centered structure which explains the stereochemical features which have been experimentally observed for this reaction. A simple diabatic model is used to rationalize the reactivity pattern that characterizes the Simmons-Smith cyclopropanation and the different behavior observed for the reaction between singlet methylene (CH2)-C-1 and olefins.