SMILES bracket symbol

Global SMILES attributes were defined as (%,0,l,2,3)-code of six symbols si, s2, s3, s4, s5, and s6. The first symbol (si) is % . This symbol indicates this SMILES attribute is global. The s2 is descriptor of a presence of fluorine 0 means F symbol is absent in the SMILES 1 means there is one F symbol 2 means there are two F symbols finally 3 means that there are three or more F symbols in the SMILES the s3, s4, s5 are the same descriptors for Cl , Br , and O , respectively the s6 is the descriptor for the ( symbol in the SMILES. The brackets are tools to reflect the branching of molecular skeleton (Weininger, 1988, 1990 Weininger et al., 1989). Thus, ( and ) are indicators of the same phenomenon These SMILES attributes (i.e., brackets) have common correlation weight. 

These simple rules allow very rapid encoding of most chemical structures and need only a few simple additions to cope with other atoms, charges, isomers, etc. Specification of atoms not in the organic subset, for example, is coded by use of an atomic symbol within square brackets. An extensive description of the SMILES system is given by Weininger and Weininger (1990). 

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