In 2012 the Nobel Prize in Chemistry was awarded to three researchers for their development in the 1970s of computer programs for simulating complex chemical reactions that seem to take place instantaneously. Since then, simulations have become a primary means for representing, discovering, and understanding the detailed processes taking place in such reactions, which are not amenable to either theoretical deduction or direct experimental investigation. Furthermore, the most compelling sense of understanding comes from visual images that follow the simulation in time, most dramatically as films depicting the process of bond formation. Such a film is something like a documentary that aims to depict a historical development. As such it requires an accompanying narration of what is happening. It typically includes a beginning “scenario” that identifies the actors (molecules), their significance, relevant properties, and environment. Then a developmental narrative is generated in parallel with the simulation. In many cases the final product of the reaction is already known and interest focuses on the stages of the reaction process, its variability, and particularly on unknown and unexpected features that the simulation throws up. When all works well, the narrated simulation provides an explanation of the reaction process as, effectively, a historical narrative. The paper will explore that characterization.

M. Norton Wise is Distinguished Professor, Department of History and Institute for Society and Genetics, UCLA. He has published widely in the history of the physical sciences in the 19^th and 20^th centuries. He is best known for studies of the co-evolution of political economy and physics, focusing on working machines in the emergence of energy physics. More recently he has investigated the changing character of explanation in contemporary natural science. Representative publications are the edited volumes Growing Explanations (2004) and (with A. Creager and E. Lunbeck) Science without Laws: Model Systems, Cases, Exemplary Narratives (2007).