Each TS structure was characterized by verification of the presence of a single imaginary frequency with a normal mode consistent with the S3→S4 steps. Evaluation of the product structure (step S4) was performed as a regular geometry optimization, starting from a slightly distorted geometry of the TS along the normal mode with the imaginary frequency; these calculations were only used to verify the reaction mechanism. The energies were not corrected for zero-point vibrational or free energy contributions, because the focus is not to reproduce accurate energetic results, e.g., for the comparison of rate constants to experiment. Such corrections would significantly increase the computational effort for the Si-12Å clusters without changing the underlying analysis of the data. All calculations were performed in vacuo using a development version of the GAUSSIAN suite of programs. 3 Results