A reactive flux correlation function formalism for the calculation of rate constants for mixed quantum-classical systems undergoing nonadiabatic dynamics is presented. The linear response formalism accounts for the stationarity of the equilibrium density under quantum-classical dynamics and expresses the rate constant in terms of an ensemble of surface-hopping trajectories. Calculations are carried out on a model two-level system coupled to a nonlinear oscillator which is in turn coupled to a harmonic heat bath. Relevant microscopic species variables for this system include two stable states, corresponding to the ground state adiabatic surface, as well as another species corresponding to the excited state surface. The time-dependent rate constants for the model are evaluated in the adiabatic limit, where the dynamics is confined to the ground Born-Oppenheimer surface, and these results are compared with calculations that account for nonadiabatic transitions among the system states. (C) 2003 American Institute of Physics.