The chemical perception framework assigns parameters via standard chemical substructure queries implemented in the SMARTS language. This removes many redundancies, for example in equivalent parameters that would otherwise be applied to different combinations of atom types, and allows the OpenFF line of FFs (Parsley , Sage , ...) to be very compact without sacrificing accuracy (Table ). Given the hierarchical nature of these FFs, their extension becomes trivial. More specific substructure queries can be introduced for problematic areas of chemistry without affecting the more general, transferable parameters.