Stochastic dynamics in irreversible nonequilibrium environments. 1. The fluctuation-dissipation relation

A generalization of the generalized Langevin equation (stochastic dynamics) is introduced in order to model chemical reactions which take place in cha nging environments. The friction kernel representing the solvent response i s given a non-stationary form with respect to which the instantaneous rando m solvent force satisfies a natural generalization of the fluctuation-dissi pation relation. Theoretical considerations, as well as numerical simulatio ns, show that the dynamics of this construction satisfy the equipartition t heorem beyond its equilibrium limits.