Excited State-Specific CASSCF Theory for the Torsion of Ethylene

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
have yet to be fully determined. We investigate the performance of
the 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 optimizing the orbitals for each excited state significantly
improves the spatial diffusivity of the wave function. However, the
incorrect ordering of state-specific solutions 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 realize practical applications
of state-specific electronic structure theory for computational photochemistry.