The chemical perception trees sampled in both SMARTY and SMIRKY were scored against the typing in existing force fields (Section 2.1.1). For SMARTY, we used a traditional indirect chemical perception force field specification as the reference, namely, AMBER's parm99 with the parm@Frosst 97 extension. For SMIRKY, we used the direct chemical perception force field smirnoff99Frosst, which was generated by hand to closely follow the parameters in the parm99/parm@Frosst force field.