Excited state-specific CASSCF theory for the torsion of ethylene

Abstract: State-specific complete active space self-consistent field (SS-CASSCF) theory has emerged as a promising route to accurately predict electronically excited energy surfaces away from molecular equilibria. However, its accuracy and practicality for chemical systems of photochemical interest has yet to be fully determined. We investigate the performance of SS-CASSCF theory for the low-lying ground and excited states in the double bond rotation of ethylene. We show that state-specific approximations with a minimal (2e, 2o) active space provide comparable accuracy to state-averaged calculations with much larger active spaces, while optimising the orbitals for each excited state significantly improves the spatial diffusivity of the wave function. However, the unbalanced post-CASSCF dynamic correlation in valence and Rydberg excitations, or the use of a non-diffuse basis set, causes excited state solutions to coalesce and disappear, creating unphysical discontinuities in the potential energy surface. Our findings highlight the theoretical challenges that must be overcome to realise practical applications of state-specific electronic structure theory for computational photochemistry.