The insertion reaction of zinc into the C-I bond of CH2I2 and subsequent cyclopropanation reactions with CH2CH2 have been investigated using B3LYP level density functional theory calculations. The Simmons-Smith cyclopropanation reaction of olefins does not proceed easily due to the relatively large barriers on the insertion and cyclopropanation pathways. The computed results indicate that the IZnCH2I molecule is the active reagent in the Simmons-Smith reaction. This is consistent with the IZnCH2I reactive species being generated from diiodomethane and a Zn-Cu couple as proposed by several other research groups. The Simmons-Smith IZnCH2I carbenoid and CH2I-I carbenoid cyclopropanation reactions with olefins are compared. The reactions of olefins with the radicals from the decomposition of the IZnCH2I and CH2I-I species were also compared. We found that the chemical reactivity of the carbenoid species is dependent on its electrophilic behavior, steric effects, the leaving group character and the mechanism of the cyclopropanation reactions.