An Approach to Estimate the Binding Energy of Interstellar Species

One of the major obstacles to accurately modeling the interstellar chemistry is inadequate knowledge of the binding energy (BE) of interstellar species with dust grains. In denser regions of molecular clouds, where very complex chemistry is active, interstellar dust is predominantly covered by H2O molecules, thus it is essential to know the interaction of gas-phase species with water ice to trace realistic physical and chemical processes. To this end, we consider water (cluster) ice to calculate the BE of several atoms, molecules, and radicals of astrochemical interest. Systematic studies have been carried out to come up with a relatively more accurate BE of astrophysically relevant species on water ice. We increase the size of the water cluster methodically to capture the realistic situation. Sequentially, one, three, four, five, and six water molecules are considered to represent water ice analogs in increasing order of complexity. We note that for most of the species considered here, as we increase the cluster size, our calculated BE value starts to converge toward the experimentally obtained value. More specifically, our computed results with the water c-pentamer (average deviation from experiment similar to +/- 15.8%) and c-hexamer (chair) (average deviation from experiment similar to +/- 16.7%) configurations are found to be nearer to an experimentally obtained value other than the value found for the water clusters we consider.