The discovery of effective hydrogen storage materials is fundamental for the progress of a clean energy economy. Ammonia borane (H3 BNH3 , AB) 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 ab-initio molecular dynamics and metadynamics to study the early stages of AB dehydrogenation. We show that the formation of ammonia diborane (H3 NBH2 (μ-H)BH3 ) leads to the release of NH4 + , which in turn triggers an autocatalytic H2 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.