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

Theoretical studies of photochemical processes require a description of theenergy surfaces of excited electronic states, especially near degeneracies, where transitionsbetween states are most likely. Systems relevant to photochemical applications are typically toolarge for high-level multireference methods, and while time-dependent density functionaltheory (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 andpyramidal distortion in ethylene, the quintessential model for photochemical studies. Byallowing for symmetry breaking, the calculated energy surfaces exhibit the correct topologyaround the twisted-pyramidalized conical intersection even when using a semilocal functionalapproximation, and by including explicit self-interaction correction, the torsional energy curvesare in close agreement with published multireference results. Thefindings of the present workpoint to the possibility of using a single determinant time-independent density functionalapproach to simulate nonadiabatic dynamics, even for large systems where multireferencemethods are impractical and TDDFT is often not accurate enough.

Automated summary

A variational, time-independent density functional approach is used to accurately describe the energy surfaces of twisting and pyramidal distortion in ethylene, even with the use of a semilocal functional approximation. The findings suggest the possibility of using a single determinant time-independent density functional approach for simulating nonadiabatic dynamics in large systems.