  Urea is a prebiotic molecule that has been detected in few sources of the
interstellar medium (ISM) and in Murchison meteorite. Being stable against
ultraviolet radiation and high-energy electron bombardment, urea is expected to
be present in interstellar ices. Theoretical and experimental studies suggest
that isocyanic acid (HNCO) and formamide (NH$_2$CHO) are possible precursors of
urea. However, uncertainties still exist regarding its formation routes.
Previous computational works characterised urea formation in the gas phase or
in presence of few water molecules by reaction of formamide with
nitrogen-bearing species. In this work, we investigated the reaction of HNCO +
NH$_3$ on an 18 water molecules ice cluster model mimicking interstellar ice
mantles by means of quantum chemical computations. We characterised different
mechanisms involving both closed-shell and open-shell species at
B3LYP-D3(BJ)/ma-def2-TZVP level of theory, in which the radical-radical
H$_2$NCO + NH$_2$ coupling has been found to be the most favourable one due to
being almost barrierless. In this path, the presence of the icy surfaces is
crucial for acting as reactant concentrators/suppliers, as well as third bodies
able to dissipate the energy liberated during the urea formation.
