  Understanding the formation of molecules under conditions relevant to
interstellar chemistry is fundamental to characterize the chemical evolution of
the universe. Using reactive molecular dynamics simulations with model or
high-quality potential energy surfaces provides a means to specifically and
quantitatively probe individual reaction channels at a molecular level. The
formation of CO$_2$ from collision of CO($^1 \Sigma$) and O($^1$D) is
characterized on amorphous solid water (ASW) under conditions characteristic in
cold molecular clouds. Recombination takes place on the sub-nanosecond time
scale and internal energy redistribution leads to stabilization of the product
with CO$_2$ remaining adsorbed on the ASW. Using a high-level potential energy
surface, formation into and stabilization of CO$_2$ and COO is observed.
