Figure 1.

The objective of the retrosynthesis game is to synthesize the target product m₀ from available substrates by way of a synthesis tree that minimizes the cost function. Molecules and reactions are illustrated by circles and squares, respectively. Starting from the target, a reaction (yellow) is selected according to a policy π(r₀|m₀) that links m₀ with precursors m₁, m₂, m₃. The gray squares leading to m₀ illustrate the other potential reactions in . The game continues one move at a time reducing intermediate molecules (blue) until there are only substrates remaining, or until a maximum depth of 10 is reached. Dead-end molecules (green), for which no reactions are possible, are assigned a cost penalty of 100, while molecules at maximum depth (purple) are assigned a cost penalty of 10. Commercially available substrates (red) are assigned zero cost. The synthesis cost of the product may be computed according to eq 1 only on completion of the game. Here, the sampled pathway leading to the target (red arrows) has a cost of 5.