TEST OF TRAJECTORY SURFACE HOPPING AGAINST ACCURATE QUANTUM DYNAMICS FOR AN ELECTRONICALLY NONADIABATIC CHEMICAL-REACTION

This paper presents the first test of the popular trajectory surface-h opping (TSH) method against accurate three-dimensional quantum mechani cs for a reactive system. The system considered is a model system in w hich an excited atom with an excitation energy of 0.76 eV reacts with or is quenched by the H-2 molecule. The electronically nonadiabatic co llisions occur primarily near a conical intersection of an exciplex wi th a repulsive ground state. The accurate quantal results are calculat ed using the outgoing wave variational principle in an electronically diabatic representation. Four variants of the TSH method are tested, d iffering in the criteria for hopping and the component of momentum tha t is adjusted in order to conserve energy when a hop occurs. Coupling between the ground and excited surface occurs primarily in the vicinit y of a conical intersection and is mediated by an exciplex found on th e upper surface. We find that the overall TSH quenching probabilities are in good agreement with quantum mechanical results, but the branchi ng ratios between reactive and nonreactive trajectories and many of th e state-selected results are poorly reproduced by trajectory calculati ons. The agreement between trajectory surface hopping and quantal resu lts is on average worse for the relatively more ''quantum mechanical'' j = 0 initial state and M + H-2 quenching process and better for the relatively more ''classical'' j = 2 initial state and MH + H' reactive process. We also perform a statistical calculation of overall quenchi ng probability and unimolecular rate of the nonadiabatic decay of the exciplex. We find that only about 10 % of trajectories can be describe d as ''statistical'' and that statistical calculation overestimates th e total quenching rate significantly.