  The formation of the interstellar complex organic molecules (iCOMs) is a hot
topic in astrochemistry. One of the main paradigms trying to reproduce the
observations postulates that iCOMs are formed on the ice mantles covering the
interstellar dust grains as a result of radical--radical coupling reactions.
  We investigate iCOMs formation on the icy surfaces by means of computational
quantum mechanical methods. In particular, we study the coupling and direct
hydrogen abstraction reactions involving the CH$_3$ + X systems (X = NH$_2$,
CH$_3$, HCO, CH$_3$O, CH$_2$OH) and HCO + Y (Y = HCO, CH$_3$O, CH$_2$OH), plus
the CH$_2$OH + CH$_2$OH and CH$_3$O + CH$_3$O systems.
  We computed the activation energy barriers of these reactions as well as the
binding energies of all the studied radicals, by means of density functional
theory (DFT) calculations on two ice water models, made of 33 and 18 water
molecules. Then, we estimated the efficiency of each reaction using the
reaction activation, desorption and diffusion energies and derived kinetics
with the Eyring equations.
  We find that radical--radical chemistry on surfaces is not as straightforward
as usually assumed. In some cases, direct H abstraction reactions can compete
with radical--radical couplings, while in others they may contain large
activation energies. Specifically, we found that (i) ethane, methylamine and
ethylene glycol are the only possible products of the relevant radical--radical
reactions; (ii) glyoxal, methyl formate, glycolaldehyde, formamide, dimethyl
ether and ethanol formation is likely in competition with the respective
H-abstraction products, and (iii) acetaldehyde and dimethyl peroxide do not
seem a likely grain surface products.
