  We introduce a simple method for characterizing reactive pathways in quantum
systems. Flux auto- correlation and cross-correlation functions are employed to
develop a quantitative measure of dynamical coupling in quantum transition
events, such as reactive tunneling and resonant energy transfer. We utilize the
method to study condensed-phase proton-coupled electron transfer (PCET)
reactions and to determine the relative importance of competing concerted and
sequential reaction pathways. Results presented here include numerically exact
quantum dynamics simulations for model condensed-phase PCET reactions. This
work demonstrates the applicability of the new method for the analysis of both
approximate and exact quantum dynamics simulations.
