bond symbol :, e.g. C:1:C:C:C:C:C1, or Most commonly, by writing the constituent B, C, N, O, P and S atoms in lower-case forms b, c, n, o, p and s, respectively. In the latter case, bonds between two aromatic atoms are assumed (if not explicitly shown) to be aromatic bonds. Thus, benzene, pyridine and furan can be represented respectively by the SMILES c1ccccc1, n1ccccc1 and o1cccc1. Aromatic nitrogen bonded to hydrogen, as found in pyrrole must be represented as [nH]; thus imidazole is written in SMILES notation as n1c[nH]cc1. When aromatic atoms are singly bonded to each other, such as in biphenyl, a single bond must be shown explicitly: c1ccccc1-c2ccccc2. This is one of the few cases where the single bond symbol - is required. (In fact, most SMILES software can correctly infer that the bond between the two rings cannot be aromatic and so will accept the nonstandard form c1ccccc1c2ccccc2.) The Daylight and OpenEye algorithms for generating canonical SMILES differ in their treatment of aromaticity. Branching Branches are described with parentheses, as in CCC(=O)O for propionic acid and FC(F)F for fluoroform. The first atom within the parentheses, and the first atom after the parenthesized group, are both bonded to the same branch point atom. The bond symbol must appear inside the parentheses; outside (E.g.: CCC=(O)O) is invalid. Substituted rings can be written with the branching point in the ring as illustrated by the SMILES COc(c1)cccc1C#N (see depiction) and COc(cc1)ccc1C#N (see depiction) which encode the 3 and 4-cyanoanisole isomers. Writing SMILES for substituted rings in this way can make them more human-readable. Branches may be written in any order. For example, bromochlorodifluoromethane may be written as FC(Br)(Cl)F, BrC(F)(F)Cl, C(F)(Cl)(F)Br, or the like. Generally, a SMILES form is easiest to read if the simpler branch comes first, with the final, unparenthesized portion being the most complex. The only caveats to such rearrangements are: If ring numbers are reused, they are paired according to their order of appearance in the SMILES string. Some adjustments may be required to preserve the correct pairing. If stereochemistry is specified, adjustments must be made; see below. The one form of branch which does not require parentheses are ring-closing bonds. Choosing ring-closing bonds appropriately can reduce the number of parentheses required. For example, toluene is normally written as Cc1ccccc1 or c1ccccc1C, avoiding the parentheses required if written as c1cc(C)ccc1 or c1cc(ccc1)C. Stereochemistry SMILES permits, but does not