Perception Algorithm for

The SSSR is a fundamental basis containing the fx smallest simple-cycles in a structure. The majority of non-chemical and many chemical applications use the SSSR as the ring set since fx can be calculated easily and the mathematical foundations of a fundamental basis are well understood. The SSSR is still the most important and widely used of all ring sets. As with the set of all cycles, algorithms exist that use each of the methods outlined in Section 3. Since the 1989 review, five improved SSSR perception algorithms for chemical structure applications have been published. ... 

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Rucker, G. and Rucker, C. (1990) Computer perception of constitutional (topological) symmetry TOPSYM, a fast algorithm for partitioning atoms and pairwise relations among atoms into equivalence classes. /. Chem. Inf. Comput. Sci., 30, 187-191. 

Chen L, Robien W. MCSS a new algorithm for perception of maximal common substructures and its application to NMR spectral studies. 1. The algorithm. J Chem Inf Comput Sci 1992 32 501-506. 

For tautomers, the ideal situation is to store one preferred tautomeric form and register other forms as synonyms of this preferred one. However, there is no universally accepted algorithm for tautomer generation, and each registration system will have its own implementation. There will be cases where the system perceives two structures to be tautomers but the chemist knows better and wants to store them separately. Conversely, due to limitations in the tautomer perception algorithm, two tautomeric forms may not be recognised as such. A practical registration system must therefore allow chemists to override the system-provided default. 

In devising conventions, we have always had to bear in mind the generic nature of the input, which results in an enormous volume of information. Because many of the input structures are generic, and in complex generics the permutations are so numerous, we cannot rely, as CAS does, on computer perception of structural features such as tautomerism, even if the algorithms for this could work reliably. 

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