Département de chimie moléculaire

The development of analytic-gradient methodol. for excited states within conventional time-dependent d.-functional theory (TDDFT) would seem to offer a relatively inexpensive alternative to better established quantum-chem. approaches for the modeling of photochem. reactions. However, even though TDDFT is formally exact, practical calcns. involve the use of approx. functional, in particular the TDDFT adiabatic approxn., the use of which in photochem. applications must be further validated. Here, we investigate the prototypical case of the sym. CC ring opening of oxirane. We demonstrate by direct comparison with the results of high-quality quantum Monte Carlo calcns. that, far from being an approxn. on TDDFT, the Tamm-Dancoff approxn. 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. [on SciFinder(R)]