Statistical study of uncertainties in the diffusion rate of species on
  interstellar ice and its impact on chemical model predictions
  Context: Diffusion of species on the dust surface is a key process for
determining the chemical composition of interstellar ices. On the dust surface,
adsorbed species diffuse from one potential well to another and react with
other adsorbed reactants, resulting in the formation of simple and complex
molecules. Aims: We study the impact on the abundances of the species simulated
by the chemical codes by considering the uncertainties in the diffusion energy
of adsorbed species. We aim to limit the uncertainties in the abundances as
calculated by chemical codes by identifying the surface species that result in
a larger error because of the uncertainties in their diffusion energy. Methods:
We ran various cases with 2000 to 10000 simulations in each case and varied the
diffusion energies of some or all surface species randomly. We calculated
Pearson correlation coefficients between the abundances and the ratio of
diffusion to binding energy of adsorbed species. We identified the species that
introduce maximum uncertainty in the ice and gas-phase abundances. With these
species we ran three sets, with 2000 simulations in each, to quantify the
uncertainties they introduce. Results: We present the abundances of various
molecules in the gas phase and also on the dust surface at different time
intervals during the simulation. We show which species produce a large
uncertainty in the abundances. We sorted species into different groups in
accordance with their importance in propagating uncertainty in the chemical
network. Conclusions: We show that CO, H$_2$, O, N, and CH$_3$ are the key
species for uncertainties in the abundances, while CH$_2$, HCO, S and O$_2$
come next, followed by NO, HS, and CH. We also show that by limiting the
uncertainties in the ratio of diffusion to binding energy of these species, we
can eliminate the uncertainties in the gas-phase abundances of almost all the
species.
