The ideal evaluation of predicted reactions would be through experimental validation . Given that each model in our case study predicts 10 reactions for 5007 targets, this validation is unfeasible. To provide an ad-hoc replacement for physical validation, Schwaller et al. employed a forward synthesis model to predict the reaction outcome for each reactant set i ∈ {1, . . . , k}, given a target molecule t (Figure ). For each precursor set i, n possible reaction outcomes p ∈ P , where P = {p1, . . . , pn}, are generated.