We are interested in the reaction probability: the probability that two entities interact during a time step given that they can interact with reaction rate k, their diffusion rates are D1 and D2 and they start a distance d apart at the beginning of the time interval Δt. This probability is illustrated in Figure 1C. The reaction between two diffusing particles can be considered to occur in two steps: firstly the encounter of the two entities through diffusion, followed by the actual chemical reaction. Let us consider two freely diffusing chemical entities, A and B, starting a distance d0 from each other at time t0. At any time t later the rate κAB (t|d0, t0) of the reaction between entities A and B can be expressed as: