# When can a Computer Simulation act as Substitute for an Experiment? A Case-Study from Chemisty

Johannes Kästner and Eckhart Arnold

#### 3.4.1 Abstractions

First of all, the simulation uses a simplified model to capture the essence of the chemical reaction that is considered to be most relevant for $H_2$-enrichment in outer space, namely the chemisorption of H and D on benzene. Benzene is the simplest aromatic hydrocarbon and it is expected that the results of the simulation regarding the reaction rate under consideration and the tunneling effect will not fundamentally differ for other polycyclic aromatic hydrocarbons, an assumption meanwhile confirmed by additional simulations (Goumans 2011).

Furthermore, only the first part of the reaction, namely the chemisorption of H or D on benzene, is simulated but not the addition of the second H or D which would complete the formation of the $H_2$, HD, or D$_2$ . The rationale for this abstraction lies in the fact that the “addition of a second H or D atom is barrier-less para to a chemisorbed H on the edge of a polyaromatic hydrocarbon” (Goumans/Kaestner 2010, p. 7351), wherefore it is the chemisorption of the first H atom that is rate-limiting.