Fig. 5. Using MCTS to find short synthesis pathways using green solvents. (a) Assigning scores for solvents in the solvent database. (b) Using the prediction of the solvent prediction model to define the reaction solvent score. The suggested top three solvents are shown with the probabilities listed and solvent scores in parentheses. The reaction solvent score (RSS) is defined as the weighted average of the top three solvent scores. (c) Converting the reactions solvent score to reaction solvent penalty (R solvent penalty), then defining the compound solvent score using the R solvent penalty. The compound solvent score (CSS) is defined as the maximal cumulated RSS in a valid pathway. The greenest route for a compound is the path which lead to the CSS. CSS is a function of tree expander and the root compound. It is essential to optimize the tree expander so that CSS can be optimized.