Floating in Space: How to Treat the Weak Interaction between CO Molecules in Interstellar Ices

In the interstellarmedium, six molecules have been conclusivelydetected in the solid state in interstellar ices, and a few dozenhave been hypothesized and modeled to be present in the solid stateas well. The icy mantles covering micrometer-sized dust grains are,in fact, thought to be at the core of complex molecule formation asa consequence of the local high density of molecules that are simultaneouslyadsorbed. From a structural perspective, the icy mantle is consideredto be layered, with an amorphous water-rich inner layer surroundingthe dust grain, covered by an amorphous CO-rich outer layer. Moreover,recent studies have suggested that the CO-rich layer might be crystallineand possibly even be segregated as a single crystal atop the ice mantle.If so, there are far-reaching consequences for the formation of morecomplex organic molecules, such as methanol and sugars, that use COas a backbone. Validation of these claims requires further investigation,in particular on acquiring atomistic insight into surface processes,such as adsorption, diffusion, and reactivity on CO ices. Here, wepresent the first detailed computational study toward treating theweak interaction of (pure) CO ices. We provide a benchmark of theperformance of various density functional theory methods in treatingthe binding of pure CO ices. Furthermore, we perform an atomisticand in-depth study of the binding energy of CO on amorphous and crystallineCO ices using a pair-potential-based force field. We find that COadsorption is represented by a large distribution of binding energies(200-1600 K) on amorphous CO, including a significant amountof weak binding sites (<350 K). Increasing both the cluster sizeand the number of neighbors increases the mean of the observed bindingenergy distribution. Finally, we find that CO binding energies aredominated by dispersion and, as such, exchange-correlation functionalsneed to include a treatment of dispersion to accurately simulate surfaceprocesses on CO ices. In particular, we find the omega B97M-V functionalto be a strong candidate for such simulations.