MODEL STUDIES OF NONADIABATIC DYNAMICS

Mixed quantum-classical methods are applied to an increasingly challen ging series of model problems, and their accuracy is examined. The mod els involve one light and one heavy degree of freedom, and exhibit sub stantial nonadiabatic behavior. In all of the models the coupling betw een the light and heavy particles is linear (harmonic), In addition, d ifferent external potentials are applied to the heavy particle only. T he energies of the light particle quantum states, as a function of the position of the heavy particle, define a sequence of ground and excit ed Born-Oppenheimer potential energy curves. Because the light particl e experiences a purely harmonic potential, the potential energy curves are parallel and equally spaced for all of the models. In addition, t he nonadiabatic couplings among potential energy curves persist for al l times due to the nonvanishing linear coupling between light and heav y particles. The model problems were used to test two strategies for c arrying out mixed quantum-classical dynamics in systems involving nona diabatic transitions: mean field and surface hopping. The model calcul ations reported here suggest that, in cases where linear couplings dom inate, the mean field mixed quantum-classical method displays useful a ccuracy and is robust to the introduction of anharmonic heavy-particle interactions. The model calculations also reveal special situations i n which the surface hopping approximation is inappropriate. (C) 1998 A merican Institute of Physics.