The transition state (TS) structures were obtained in three steps: (a) build a freezing string between the reactant and product geometries, (b) take the highestenergy point along the string, and (c) refine the structure using Baker's partitioned rational-function optimization (P-RFO) method. Subsequent harmonic vibrational frequency analysis was performed to confirm a local minima (no imaginary frequency) for the reactant and product or a saddle point (one imaginary frequency) for the transition state. Intrinsic reaction coordinate (IRC) calculations were carried out to connect the TS to the reactant and product structures. The MECP structure for the spin-crossing reaction was optimized using the algorithm proposed by Harvey and coworkers as implemented in the sobMECP software by Lu ().