Density functional theory was employed to calculate the adsorption/dissociation of H2 on gold surfaces, Au(111) and Au(100), and on gold particles from 0.7 (Au14) to 1.2 nm (Au29). Flat surfaces of the bulk metal were not active towards H2, but a different effect was observed in gold nanoclusters, where the hydrogen was adsorbed through a dissociative pathway. Several parameters such as the coordination of the Au atoms, ensemble effects and fluxionality of the particle were analyzed to explain the observed activity. The effect of the employed functional was also studied. The flexibility of the structure, i.e., its adaptability towards the adsorbate, plays a key role in the bonding and dissociation of H2. The interaction with hydrogen leads to drastic changes in the structure of the Au nanoparticles. Furthermore, it appears that not only low coordinated Au atoms are needed because H2 adsorption/dissociation was only observed when a cooperation between several (4) active Au atoms was allowed.