title: 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 functional, in particular the TDDFT adiabatic approximation, the use of which 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 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.
authors.affiliation: Laboratoire de Chimie Théorique, Département de Chimie Molécularie (DCM, UMR CNRS/UJF 5250), Institut de Chimie Moléculaire de Grenoble (ICMG, FR2607), Université Joseph Fourier (Grenoble I), F-38041 Grenoble Cedex 9, France.
authors.forename: Felipe
authors.initials: F
authors.lastname: Cordova
authors.forename: L Joubert
authors.initials: LJ
authors.lastname: Doriol
authors.forename: Andrei
authors.initials: A
authors.lastname: Ipatov
authors.forename: Mark E
authors.initials: ME
authors.lastname: Casida
authors.forename: Claudia
authors.initials: C
authors.lastname: Filippi
authors.forename: Alberto
authors.initials: A
authors.lastname: Vela