First principles potentials for reactions on molecular crystals: modelling the interstellar H + CO reaction

The potential energy barriers for successive hydrogenation of carbon monoxide mean that the observed interstellar abundances of small hydrogenated carbon molecules such as formaldehyde cannot be accounted for by gas phase hydrogenation reactions. Consequently these reactions are thought to occur on the surface of ice grains in the interstellar medium. Aspects of the interaction of hydrogen atoms with cold CO ice surfaces are examined here by means of molecular dynamics calculations. In order to undertake these calculations a new and flexible approach to developing potential energy surfaces for gas-surface processes (including with ions as reactants) on finite temperature molecular crystals is developed. Interaction energies are calculated by a deep learned potential from electronic structure theory calculations incorporating extended CO ice structures, including under periodic boundary conditions. Direct hydrogenation on the CO ice surface requires energies higher than those typically available in the interstellar medium. Hydrogen atoms are readily adsorbed onto ice surfaces to feed catalytic hydrogenation processes within and on the icy grain, but do not directly reflect the temperature dependence of reactivity observed experimentally.