  First-principles molecular dynamics is employed to describe the atomic
structure of amorphous SiN, a non-stoichiometric compound belonging to the
Si$_x$N$_{y}$ family. To produce the amorphous state via the cooling of the
liquid, both the Car-Parrinello and the Born-Oppenheimer approaches are
exploited to obtain a system featuring sizeable atomic mobility. At high
temperatures, due to the peculiar electronic structure of SiN, exhibiting gap
closing effects, the Car-Parrinello methodology could not be followed since
non-adiabatic effects involving the ionic and electronic degrees of freedom do
occur. This shortcoming was surmounted by resorting to the Born-Oppenheimer
approach allowing to achieve significant ionic diffusion at $T$= 2500 K. From
this highly diffusive sample, an amorphous state at room temperature was
obtained with a quenching rate of 10 K/ps. Four different models were created,
differing by their sizes and the thermal cycles. We found that the subnetwork
of atoms N has the same environment than in the stoichiometric material
Si$_3$N$_4$ since N is mostly threefold coordinated with Si. Si atoms can also
be found coordinated to four N atoms as in Si$_3$N$_4$, but a substantial
fraction of them forms homopolar bonds with one, two, three and even four Si.
Our results are not too dissimilar from former models available in the
litterature but they feature a higher statistical accuracy and refer more
precisely to room temperature as the reference thermodynamical condition for
the analysis of the structure in the amorphous state.
