The two-state reaction mechanism of CpCo(C_4H_4)with isocyanate on the triplet and singlet potential energy surfaces has been investigated at the B3LYP level.A study is described for the computation of spin-orbit coupling of triplet state of the minimal energy crossing point(CP)with their singlet states and of the zerofield splitting(ZFS)parameters of the triplet states,including the full one-and two-electron terms of the BreitPauli Hamiltonian.There are two key crossing points along this two-state reaction pathway.The first crossing point—CP2 exists near~1B.The reacting system will change its spin multiplicity from the triplet state to the singlet state near this crossing region.Although the spin-orbit coupling interaction and ZFS D-tensor of the CP2 region are very strong,the reaction system will occur the reverse intersystem crossing from T_1 to S_0.Therefore,its spin-flip efficiency may be lower.The second crossing point,CP3will again change its spin multiplicity from the singlet state to the triplet state in the Co-Cr bond activation pathway,leading to a decrease in the barrier height of~1TS(CF)from19.5 to 9.5 kcal/mol(1cal=4.182 J),and the efficiency of intersystem crossing from S_0 to T_1 is high because the larger spin-orbit coupling(SOC)matrix elements will result in the overpopulations of the three sublevels of T_1(3.30×10~(-1),3.32×10~(-1),and 3.38×10~(-1),respectively). The two-state reaction mechanism of CpCo (C_4H_4) with isocyanate on the triplet and singlet potential energy surfaces has been investigated at the B3LYP level. A study is described for the computation of spin-orbit coupling of triplet state of the minimal energy crossing point (CP) with their singlet states and of the zerofield splitting (ZFS) parameters of the triplet states, including the full one-and two-electron terms of the BreitPauli Hamiltonian. There are two key crossing points along this two-state reaction pathway. The first system point-CP2 exists near ~ 1B. The reacting system will change its spin multiplicity from the triplet state to the singlet state near this crossing region. Although the spin-orbit coupling interaction and ZFS D-tensor of the CP2 region are very strong, the reaction system will occur the reverse intersystem crossing from T_1 to S_0.Therefore, its spin-flip efficiency may be lower. The second crossing point, CP3 last again change its spin multiplicity from the singlet state to the triplet state in the Co-Cr bond activation pathway, leading to a decrease in the barrier height of ~ 1TS (CF) from 19.5 to 9.5 kcal / mol (1cal = 4.182 J), and the efficiency of intersystem crossing from S_0 to T_1 is high because the larger spin-orbit coupling (SOC) matrix elements will result in the overpopulations of the three sublevels of T_1 (3.30 × 10 -1, 3.32 × 10 -1, and 3.38 × 10 -1, respectively).