Computer simulations are playing an increasingly important role in the study of chemical and biochemical reactions in condensed phases [1, 2, 3]. For bimolecular reactions, the events leading to reaction can be separated into two steps: the initial encounter, followed by the actual reaction of the properly juxtaposed reactants. Current simulation methods allow the analysis of reactions whose rates are controlled by one or the other of these steps [1]. Here, we describe an approach that can be used for the general case. An advantage of this approach is that it allows the rigorous integration of a hierarchy of models. E.g., the encounter step can be treated by models with continuum and Brownian elements, and the reaction step by fully atomistic models.