The indicator function I(•) takes the value one or zero depending on whether the designed reactant set S belongs to a subset of purchasable compounds. The second and third terms on the right-hand side penalize the increasing number of reactants (|S|) and the size of the designed network (|G|), where τ and τ ′ determine the magnitude of penalties. By identifying x with a sufficiently high posterior probability, we predict product P and its synthetic reaction route {S, G} that satisfy design objective U * . However, computational difficulty here arises from the extremely large search space. The search space is composed of all combinations of reactants that are purchasable. The number of candidate reactants is typically of the order O(10 6 ), resulting in the cardinality of the solution space burgeoning to O(10 6×k ) when k reactants are involved in the synthetic route planning. In addition, the network topology to be explored further increases the size of the search space.