While the above described loop through stereoisomers of the ligand to find the best atomic charge distribution works well in most cases, there are a few examples where the negative charge is in fact not assigned to the coordinating atom. The sulfonate anion in OPAKAT (Figure ) is an example of the negative charge initially not being assigned to the coordinating atom. For the isolated ligand, the oxygen atoms are equivalent and only one resonance form with the charge assigned to a random oxygen is created. This is one of two post SMILES creation fixes that are done using SMARTS pattern matching. The other post fix relates to -NO 2 groups that we want to consistently be described with the same representation. Two Lewis structures obeying the octet rule can be drawn for an -NO 2 group (IXENEG, Figure ), both with two formal charges. We make sure to always use the neutral representation, adjusting the metal oxidation state accordingly.