Canonical Node Ordering
The next step uses a minor variation of the Daylight CANON algorithm to uniquely order all donor and acceptor atoms.
David Weininger, Arthur Weininger and Joseph L. Weininger, “SMILES 2: Algorithm for Unique SMILES Notation”, Journal of Chemical Information and Computer Science (JCICS), Vol. 29, No. 2, pp. 97-101, 1989.