Beyond predicting the P(active) of untested building blocks, clusters can also be used to identify groups of building blocks that are compatible. After assigning each building block to a cluster, we join the cluster results on the trisynthon data in order to get a list of three cluster IDs (corresponding to the cluster assignment for the building block at each position) for each trisynthon. Grouping by the cluster ID at each position then allows us to calculate the probability of forming an active compound for every distinct cluster combination (Figure -F, Table S6-S8). We can describe the subset of compounds where the building blocks in positions p i and p j are members of the x th cluster of p i and the y th cluster of p j as