  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.
