Automated reaction path searches for spin-forbidden reactions

Many catalytic and biomolecular reactions containing transition metals involve changes in the electronic spin state. These processes are referred to as "spin-forbidden" reactions within nonrelativistic quantum mechanics framework. To understand detailed reaction mechanisms of spin-forbidden reactions, one must characterize reaction pathways on potential energy surfaces with different spin states and then identify crossing points. Here we propose a practical computational scheme, where only the lowest mixed-spin eigenstate obtained from the diagonalization of the spin-coupled Hamiltonian matrix is used in reaction path search calculations. We applied this method to the ^6,4 FeO⁺  + H₂ → ^6,4 Fe⁺  + H₂ O, ^6,4 FeO⁺  + CH₄ → ^6,4 Fe⁺  + CH₃ OH, and ⁷ Mn⁺  + OCS → ⁵ MnS⁺  + CO reactions, for which crossings between the different spin states are known to play essential roles in the overall reaction kinetics. © 2018 Wiley Periodicals, Inc.