Physical Chemical Insight into Interstellar Grain Surface Chemistry

The darkness readily observed between the stars on a clear night sky is far from empty. In fact, a large variety of molecules has been detected in the gas phase. They make up about 99% of the total mass in the Interstellar Medium (ISM). The final 1% of mass in the ISM is brought about by dust grains that once were expelled by dying stars. I focus primarily on the cold, dense molecular medium where surface reactions lead to the build-up of `dirty' ices leading to a solid-state molecular mantle covering the micron-sized dust grains. The composition of this mantle in turn influences the surface chemistry of most other species that are formed in situ. Species accrete, diffuse, and react on the surface after which they can evaporate back into the gas phase. The interplay between these processes determines which molecules are formed, where, and if they are astronomically observable, either in the solid or gas phase. Confirmation or exclusion of reaction pathways is possible experimentally, however, it is nearly impossible to quantitatively disentangle the relative importance on realistic amorphous ices. In this talk I will discuss three recent computational chemistry inspired projects: * H₂ reactivity during CH₄ formation in early phases of star formation * Collective vibrations in amorphous water ice for energy dissipation * PAH adsorption on bare forsterite grains and the role of defects in the mineral