  Interstellar grains are known to be important actors in the formation of
interstellar molecules such as H$_2$, water, ammonia, and methanol. It has been
suggested that the so-called interstellar complex organic molecules (iCOMs) are
also formed on the interstellar grain icy surfaces by the combination of
radicals via reactions assumed to have an efficiency equal to unity. In this
work, we aim to investigate the robustness or weakness of this assumption by
considering the case of acetaldehyde (CH$_3$CHO) as a starting study case. In
the literature, it has been postulated that acetaldehyde is formed on the icy
surfaces via the combination of HCO and CH$_3$. Here we report new theoretical
computations on the efficiency of its formation.
  To this end, we coupled quantum chemical calculations of the energetics and
kinetics of the reaction CH$_3$ + HCO, which can lead to the formation of
CH$_3$CHO or CO + CH$_4$. Specifically, we combined reaction kinetics computed
with the Rice-Ramsperger-Kassel-Marcus (RRKM) theory (tunneling included)
method with diffusion and desorption competitive channels. We provide the
results of our computations in the format used by astrochemical models to
facilitate their exploitation.
  Our new computations indicate that the efficiency of acetaldehyde formation
on the icy surfaces is a complex function of the temperature and, more
importantly, of the assumed diffusion over binding energy ratio $f$ of the
CH$_3$ radical. If the ratio $f$ is $\geq$0.4, the efficiency is equal to unity
in the range where the reaction can occur, namely between 12 and 30 K. However,
if $f$ is smaller, the efficiency dramatically crashes: with $f$=0.3, it is at
most 0.01. In addition, the formation of acetaldehyde is always in competition
with that of CO + CH$_4$.
