Due to the high correlation between the proportions of these remaining cluster species, the exact ratios of these species as the reactions progressed could not be confidently determined using this method. However, from the cluster proportions obtained from this refinement [Figure ], in all of the in situ reactions, the growth in the concentration of the double cluster occurs alongside a reduction in the amount of single cluster, and there is no delay between the loss of single cluster and the formation of the double cluster. This suggests that the double cluster forms by the direct combination of single clusters, rather than forming directly from the tetramer (a pathway not involving the single cluster) or requiring the breakdown of single clusters into lower nuclearity clusters prior to the formation of the double cluster.