The radiationless decay of T-1 norbornene to the singlet ground state is st
udied using density-functional and ab initio CASSCF calculations of the pot
ential energy surface crossing and of the spin-orbit coupling. The rate of
decay is predicted using two approximate multi-dimensional non-adiabatic me
thods, one of which is based on Fermi's Golden Rule, and the other is a ver
sion of RRKM theory adapted for non-adiabatic processes. Unlike a previous
Landau-Zener treatment of this process by some of us [Chem. Phys. Lett. 287
(1998) 602-607], both methods correctly predict a short lifetime for the t
riplet excited state, in reasonable agreement with experimental data. This
underlines the importance of tunnelling in non-radiative relaxation process
es. (C) 2000 Elsevier Science B.V. All rights reserved.