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Molecular clouds are the cold regions of the Milky Way where stars form. Theyare enriched by rather complex molecules. Many of these molecules are believedto be synthesized on the icy surfaces of the interstellar submicron-sized dustgrains that permeate the Galaxy. At 10 K thermal desorption is ineffcient and,therefore, why these molecules are found in the cold gas has tantalizedastronomers for years. The assumption of the current models, called chemicaldesorption, is that the molecule formation energy released by the chemicalreaction at the grain surface is partially absorbed by the grain and theremaining one causes the ejection of the newly formed molecule into the gas.Here we report an accurate ab-initio molecular dynamics simulations aimed tostudy the fate of the energy released by the first reaction of the H additionchain on CO, CO + H $\rightarrow$ HCO, occurring on a crystalline ice surfacemodel. We show that about 90% of the HCO formation energy is injected towardsthe ice in the first picosecond, leaving HCO with an energy content (10-15 kJmol$^{-1}$) more than a factor two lower than its adsorption energy (30 kJmol$^{-1}$). As a result, in agreement with laboratory experiments, we concludethat chemical desorption is ineffcient for this specific system, namely H + COon crystalline ice. We suspect this behavior to be quite general when dealingwith hydrogen bonds, which are responsible of both the cohesive energy of theice mantle and the interaction with adsorbates, as the HCO radical, even thoughad hoc simulations are needed to draw specific conclusions on other systems.

Chemical desorption versus energy dissipation: insights from ab-initio molecular dynamics of HCO formation