The reaction rate κAB (t|d0, t0) can be integrated over a simulation timestep Δt to provide the probability of at least one reaction taking place in that timestep. We are interested in the probability of at least one event taking place during the time interval Δt, i.e. 1 - P(no event during Δt). The process under consideration is a Poisson process with a time dependent rate of the event taking place. Given the rate κ(t) of an event taking place at a time t, the probability, PAB, of at least one reaction taking place during that time interval takes the general form [24,25]: