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Theoretical studies of photochemical processes require a description of theenergy surfaces of excited electronic states, especially near degeneracies,where transitions between states are most likely. Systems relevant tophotochemical applications are typically too large for high-levelmultireference methods, and while time-dependent density functional theory(TDDFT) is efficient, it can fail to provide the required accuracy. Avariational, time-independent density functional approach is applied to thetwisting of the double bond and pyramidal distortion in ethylene, thequintessential model for photochemical studies. By allowing for symmetrybreaking, the calculated energy surfaces exhibit the correct topology aroundthe twisted-pyramidalized conical intersection even when using a semilocalfunctional approximation, and by including explicit self-interactioncorrection, the torsional energy curves are in close agreement with publishedmultireference results. The findings of the present work point to thepossibility of using a single determinant time-independent density functionalapproach to simulate nonadiabatic dynamics, even for large systems wheremultireference methods are impractical and TDDFT is often not accurate enough.

Variational Density Functional Calculations of Excited States: Conical Intersection and Avoided Crossing in Ethylene Bond Twisting