  The discovery of effective hydrogen storage materials is fundamental for the
progress of a clean energy economy. Ammonia borane ($\mathrm{H_3BNH_3}$) has
attracted great interest as a promising candidate but the reaction path that
leads from its solid phase to hydrogen release is not yet fully understood. To
address the need for insights in the atomistic details of such a complex solid
state process, in this work we use \textit{ab-initio} molecular dynamics and
metadynamics to study the early stages of AB dehydrogenation. We show that the
formation of ammonia diborane ($\mathrm{H_3NBH_2(}$$\mu$$\mathrm{-H)BH_3}$)
leads to the release of $\mathrm{NH_4^+}$, which in turn triggers an
autocatalytic $\mathrm{H_2}$ production cycle. Our calculations provide a model
for how complex solid state reactions can be theoretically investigated and
rely upon the presence of multiple ammonia borane molecules, as substantiated
by standard quantum-mechanical simulations on a cluster.
