We have recently studied the excited states of the simplest aryl olefin styrene (Bearpark, M. J.; Olivucci, M.; Wilsey, S.; Bernardi, F.; Robb, M. A. J. Am. Chem. Soc. 1995, 117, 6944−6953) at the CASSCF / 4-31G level of theory. Full geometry optimization was shown to be essential in characterizing decay funnels for internal conversion (activated) and intersystem crossing (activationless) processes in this molecule. Here, we demonstrate that the CASSCF potential energy surfaces for styrene excited states can be simulated to an acceptable level of accuracy using a hybrid molecular mechanics−valence bond method (MMVB), which is many orders of magnitude less expensive computationally than CASSCF. The nonradiative deactivation of styrene and indene from S1 is compared. Because ethylene torsions are restricted, the mechanism proposed for styrene S1 decay (involving S1/T2, T2/T1, and T1/S0 surface crossings) is much less likely to occur in indene. The existence of both S2/S1 and S1/S0 conical intersections is c...