if Filters(p) and MaximumTanimotoSimilarity(p, library) < T then library ← library ∪ {p} end if end while return library end procedure ultra-large virtual chemical spaces. If the similarity search identifies an exact match or close analogs (e.g., tautomers), the compound can be considered synthesizable using the available building blocks and chemical reactions. Importantly, SpaceLight is exact in the sense that a compound identical to the query achieves a maximum similarity score of 1.0. However, in rare cases (2 out of 10,000; see Table and Figure ), compounds enumerated using Algorithm 1 yielded similarity scores below 1.0 when searched against our chemical space. We suppose that these false negatives are due to inconsistencies in the handling of chemical reactions between RDKit and SpaceLight. Despite this flaw, given its extreme infrequency, we consider SpaceLight to be sufficiently reliable for assessing the presence of compounds within our chemical space.