Revisiting the reactivity between HCO and CH$_3$ on interstellar grain
  surfaces
  Formation of interstellar complex organic molecules is currently thought to
be dominated by the barrierless coupling between radicals on the interstellar
icy grain surfaces. Previous standard DFT results on the reactivity between
CH$_3$ and HCO on amorphous water surfaces, showed that formation of CH$_4$ +
CO by H transfer from HCO to CH$_3$ assisted by water molecules of the ice was
the dominant channel. However, the adopted description of the electronic
structure of the biradical (i.e., CH$_3$/HCO) system was inadequate (without
the broken-symmetry (BS) approach). In this work, we revisit the original
results by means of BS-DFT both in gas phase and with one water molecule
simulating the role of the ice. Results indicate that adoption of BS-DFT is
mandatory to describe properly biradical systems. In the presence of the single
water molecule, the water-assisted H transfer exhibits a high energy barrier.
In contrast, CH$_3$CHO formation is found to be barrierless. However, direct H
transfer from HCO to CH$_3$ to give CO and CH$_4$ presents a very low energy
barrier, hence being a potential competitive channel to the radical coupling
and indicating, moreover, that the physical insights ofthe original work remain
valid.
