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Table 5 Atom and bond primitives for encoding of atom types and atom environments in SMARTS

From: PubChem atom environments

Feature SMARTS encoding Special case
Atom primitives
 Element Atomic symbol Lower case indicating aromaticity
 Formal charge ±<integer> Uncharged represented as +0
 Implicit hydrogen count h<integer>  
 Explicit degree D<integer>  
 Valence v<integer>  
Bond primitives
 Single bond  
 Double bond =  
 Triple bond #  
  1. Aromatic bonds are implied between aromatic atoms unless explicitly specified otherwise.