  Theoretical studies of photochemical processes require a description of the
energy surfaces of excited electronic states, especially near degeneracies,
where transitions between states are most likely. Systems relevant to
photochemical applications are typically too large for high-level
multireference methods, and while time-dependent density functional theory
(TDDFT) is efficient, it can fail to provide the required accuracy. A
variational, time-independent density functional approach is applied to the
twisting of the double bond and pyramidal distortion in ethylene, the
quintessential model for photochemical studies. By allowing for symmetry
breaking, the calculated energy surfaces exhibit the correct topology around
the twisted-pyramidalized conical intersection even when using a semilocal
functional approximation, and by including explicit self-interaction
correction, the torsional energy curves are in close agreement with published
multireference results. The findings of the present work point to the
possibility of using a single determinant time-independent density functional
approach to simulate nonadiabatic dynamics, even for large systems where
multireference methods are impractical and TDDFT is often not accurate enough.
