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

TL;DR

This paper demonstrates that a variational, time-independent density functional approach can accurately model excited state energy surfaces and conical intersections in ethylene, offering a practical alternative for large photochemical systems.

Contribution

It introduces a symmetry-breaking variational DFT method that captures conical intersections and avoids the limitations of TDDFT for excited state calculations.

Findings

Correct topology of energy surfaces around conical intersections

Close agreement with multireference results when including self-interaction correction

Potential to simulate nonadiabatic dynamics in large systems

Abstract

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.