SubstitutionS radical tree not sufficiently exhaustive

The SubstitutionS kinetics family consists of RSR reacting with a radical species. It is currently turned off by default, but I've been using it for the model I've been working on with sulfur.

Based on the comments in the Java database, it seems that the radical tree is based off of the radical tree in H_Abstraction. However, while the H_Abstraction tree has grown, the SubstitutionS tree has not.

The two cases I've encountered were only caught because a reaction was found, but RMG was unable to find the reverse reaction because a species did not fit anywhere in the tree. I'm not sure what happens if a reactant can't be matched to a node to begin with. Does RMG just assume that it can't react in that particular family and move on?

Case 1: [S] (triplet)

• Similar to [O] (triplet), which does have a node in the tree (O_atom_triplet under Y_1centerbirad)
• Does not match anything because Y_1centerbirad only allows Cs, Cd, O
• Could be easily fixed by adding more atom types under Y_1centerbirad (H_Abstraction has Cs, Cd, CO, CS, O, S, N)

Case 2: C=C(C)[CH][S]

• Would fall under Y_rad in the H_Abstraction tree, nothing more specific applies
• Does not match anything because there is no Y_rad group in SubstitutionS
• Could be fixed by adding Y_rad group and moving some of the existing head nodes (HJ, CJ, SJ, O_rad) under it

I think the SubstitutionS radical tree should definitely be expanded/restructured. However, I'm not sure what would be the optimal way in terms of the chemistry. In theory, it could be an exact copy of the H_Abstraction radical tree, but there might be some groups that we don't want to react in SubstitutionS.

Start by comparing the SubstitutionS radical tree with the H_Abstraction radical tree and review the Java database comments referenced in the issue. Determine which atom types and groups should be supported without permitting inappropriate reactions, then verify that the two reported sulfur cases match valid nodes and that reverse reactions can be found.