Direct diabatization of electronic states by the fourfold-way: Including dynamical correlation by multi-configuration quasidegenerate perturbation theory with complete active space self-consistent-field diabatic molecular orbitals

We propose a new scheme for the direct diabatization of MC-QDPT wave functions. Our new scheme utilizes CASSCF diabatic molecular orbitals (DMOs); this is conceptually simpler than the previous approach and can lead to smoother diabatic potentials. We validated the new diabatization scheme, in comparison to CASSCF diabatization and to the original MC-QDPT diabatization scheme, for two test cases, the dissociation of LiF and the reaction of Li + FH → LiF + H. The results with our new scheme suggest that the new scheme with CASSCF DMOs would be a good choice for nonadiabatic dynamics studies in the future.