Mechanisms of the cycloaddition reactions of singlet difluorocarbene (CF(2)) to alkenes and disilene were studied using CASSCF, MR-MP2, CR-CC(2,3), and UB3LYP methods in combination with basis sets up to 6-311++G(3d,p). The CASSCF(4,4) energies suggest that the cycloadditions all follow the stepwise mechanism. However, energies calculated using the MR-MP2(4,4) and CR-CC(2,3) methods in combination with the 6-311G(d) or larger basis sets consistently show that the reactions follow a concerted mechanism. The stepwise mechanisms predicted at the CASSCF level are "artificial" because of their neglect of dynamic electron correlation effects. The importance of dynamic electron correlation in determining the mechanistic nature of the reactions is explained through knowledge of the reacting system's geometries and charges along the reaction path.