  The radicals HCO and CH$_3$ on carbon monoxide ice surfaces were simulated
using density functional theory. Their binding energy on amorphous CO ice shows
broad distributions, with approximative average values of 500 K for HCO and 200
K for CH$_3$. If they are located on the surface close to each other (3 to 4
\AA), molecular dynamics calculations based on density functional theory show
that they can form acetaldehyde (CH$_3$CHO) or CH$_4$ + CO in barrier-less
reactions, depending on the initial orientation of the molecules with respect
to each other. In some orientations, no spontaneous reactions were found, the
products remained bound to the surface. Sufficient configurational sampling,
inclusion of the vibrational zero point energy, and a thorough benchmark of the
applied electronic structure method are important to predict reliable binding
energies for such weakly interacting systems. From these results it is clear
that complex organic molecules, like acetaldehyde, can be formed by
recombination reactions of radicals on CO surfaces.
