As such, describing strong correlation in transition states has long been poised as a potential application for multiconfigurational approaches such as complete active space selfconsistent field (CASSCF) theory. This approach overcomes the difficulty of describing multiconfigurational systems by describing the state as a superposition of the possible electronic configurations in an "active space" of orbitals and electrons: |Ψ CASSCF ⟩ = n 1 n 2 ...n L C n 1 n 2 ...n L |22...n 1 n 2 ...n L 00...⟩ (1) in which n 1 n 2 ...n L enumerates the possible occupations of the L active orbitals. With a good choice of active space, all static correlation can be addressed with far fewer configurations than FCI and comparable expense to DFT. However, despite the many academic applications of these approaches in the literature, the widespread adoption of these methods for reactivity has been hindered by the challenge of choosing a consistent and adequate active space along the reaction surface. The CASSCF energy expression is given by