Troubleshooting Time-Dependent Density-Functional Theory for Photochemical Applications: Oxirane

Abstract

The development of analytic-gradient methodology for excited states within conventional time-dependent density-functional theory (TDDFT) would seem to offer a relatively inexpensive alternative to better established quantum-chemical approaches for the modeling of photochemical reactions. However, even though TDDFT is formally exact, practical calculations involve the use of approximate functionals, in particular the TDDFT adiabatic approximation, whose use in photochemical applications must be further validated. Here, we investigate the prototypical case of the symmetric CC ring opening of oxirane. We demonstrate by direct comparison with the results of high-quality quantum Monte Carlo calculations that, far from being an approximation on TDDFT, the Tamm-Dancoff approximation (TDA) is a practical necessity for avoiding triplet instabilities and singlet near instabilities, thus helping maintain energetically reasonable excited-state potential energy surfaces during bond breaking. Other difficulties one would encounter in modeling oxirane photodynamics are pointed out but none of these is likely to prevent a qualitatively correct TDDFT/TDA description of photochemistry in this prototypical molecule.

arXiv comment19 pages, 17 figures, submitted to the Journal of Chemical Physics

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• Density Functional Theory for Electronic Excited States
• Beyond the Coulson-Fischer point: Characterizing single excitation CI and TDDFT for excited states in single bond dissociations
• A hybrid approach to excited-state-specific variational Monte Carlo and doubly excited states
• Tracing potential energy surfaces of electronic excitations via their transition origins: application to Oxirane
• Analytic Gradients and Derivative Couplings for Configuration Interaction with All Single Excitations and One Double Excitation -- En Route to Nonadiabatic Dynamics