  One of the major obstacles to accurately model the interstellar chemistry is
an inadequate knowledge about the binding energy (BE) of interstellar species
with dust grains. In denser region of molecular cloud, where very complex
chemistry is active, interstellar dust is predominantly covered by H2O
molecules and thus it is essential to know the interaction of gas phase species
with water ice to trace realistic physical and chemical processes. To this
effect, 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 analogue in
increasing order of complexity. We notice that for most of the species
considered here, as we increase the cluster size, our calculated BE value
starts to converge towards the experimentally obtained value. More
specifically, our computed results with water c-pentamer (average deviation
from experiment ~ 15.8%) and c-hexamer (chair) (average deviation from
experiment ~ 16.7%) configuration are found to be more nearer as the
experimentally obtained value than other water clusters considered.
