  Physical systems with many degrees of freedom can often be understood in
terms of transitions between a small number of metastable states. For
time-homogeneous systems with short-term memory these transitions are fully
characterized by a set of rate constants. We consider the question how to
extend such a coarse-grained description to non-stationary systems and to
systems with finite memory. We identify the physical regimes in which
time-dependent rates are meaningful, and state microscopic expressions that can
be used to measure both externally time-dependent and history-dependent rates
in microscopic simulations.
