  Interstellar complex organic molecules (iCOMs) can be loosely defined as
chemical compounds with at least six atoms in which at least one is carbon. The
observations of iCOMs in star-forming regions have shown that they contain an
important fraction of carbon in a molecular form, which can be used to
synthesize more complex, even biotic molecules. Hence, iCOMs are major actors
in the increasing molecular complexity in space and they might have played a
role in the origin of terrestrial life. Understand-ing how iCOMs are formed is
relevant for predicting the ultimate organic chemistry reached in the
interstellar medium. One possibility is that they are synthesized on the
interstellar grain icy surfaces, via recombination of previously formed
radicals. The present work focuses on the reactivity of HCO with CH3/NH2 on the
grain icy sur-faces, investigated by means of quantum chemical simulations. The
goal is to carry outa systematic study using different computational approaches
and models for the icy surfaces. Specifically, DFT computations have been
bench-marked with CASPT2 and CCSD(T) methods, and the ice mantles have been
mimicked with cluster models of 1, 2, 18 and 33 H2O molecules, in which
different reaction sites have been considered. Our results indicate that the
HCO + CH3/NH2 reactions, if they actually occur, have two major competitive
channels: the formation of iCOMs CH3CHO/NH2CHO, or the formation of CO +
CH4/NH3. These two channels are either barrierless or presentrelatively low
($\leq$ 10 kJ/mol equal to about 1200 K) energy barriers. Finally, we briefly
discuss the astrophysical implications of these findings.
