In this paper we address the engineering of complex and emerging computational systems featuring situatedness, adaptivity and self-organisation, like pervasive computing applications in which humans and devices, dipped in a very mobile environment, opportunistically interact to provide and exploit information services. We adopt a meta-model in which possibly mobile, interconnected and communicating agents work according to a set of chemical-like laws. According to this view, substantiated by recent research on pervasive computing systems, we present the Alchemist simulation framework, which retains the performance of known Stochastic Simulation Algorithms for (bio)chemistry, though it is tailored to the specific features of complex and situated computational systems.