When can a Computer Simulation act as Substitute for an Experiment? A Case-Study from Chemisty
|Table of Contents|
|2 Similarities and Differences between Simulations and Experiments|
|3 Case Study: Simulation of H-2-Formation in Outer Space|
|3.1 Introductory Remarks on Simulations in Chemistry|
|3.2 The Role of Quantum Mechanics as Comprehensive Background Theory|
|3.3 The Motivation for Simulating the H-2-Formation in Outer Space|
|3.4 Modeling Techniques and their Credentials|
|4 Summary and Conclusions|
The simulation of -formation in outer space described in the following is documented in Goumans/Kaestner (2010). The purpose of this simulation is to contribute to the explanation of -enrichment in the interstellar medium. The simulation can best be described as a piece in the puzzle to explain this phenomenon. The point where the simulation study picks up the problem is defined by a number of previously established facts and existing astrochemical hypotheses:
The question that Goumans/Kaestner (2010) seek to answer is whether chemisorption of H and D atoms to polycyclic aromatic hydrocarbons (as a model for dust grains consisting of carbon), and in particular the tunneling effect, can account for the -enrichment in the interstellar medium. In order to answer this question the reaction rates of the chemisorption of H and D on benzene, the simple-most aromatic hydrocarbon, need to be determined. The reaction rates of the chemisorption of H and D on benzene can be determined experimentally only for temperatures that are much higher than those in in the interestellar medium in outer space. Therefore, the experimental determination of the reaction rate must be surrogated by numerical calculation. In the given low temperature setting the reaction rates depend crucially on the tunnel effect. If the tunneling rates can be brought into agreement with the observations and suggestions listed above, then this supports both the assumption that -formation in outer space is catalyzed by polycyclic aromatic hydrocarbons and that the tunneling effect plays a crucial role in this reaction.
In principle, the tunneling effect can also be observed experimentally, but practically this is well-nigh impossible in the given scenario, because the reaction rates are too low for experimental purposes due to the low temperatures (Goumans/Kaestner 2010, p. 7351). The time scales relevant to the interstellar medium (10 years) can not even closely be reached in experiments. The more welcome therefore is the possibility to simulate this reaction in the computer. At the same time, because no direct experimental validation of the simulation is available, more strain is put on the justification of the theoretical and technical ingredients of this simulation which will be described in the following.