Apart from ToposPro and Systre, a few alternative tools have been developed to simplify structures into topological descriptors, but none are suitable for topology identification of chemical structures at large. Indeed, they either only work on specific structures (usually zeolites) like TOTOPOL , or rely on a non-unique representation of the structure (such as a tiling ) like the T-ring representation , or because their output is not unique for each input structure, like the Single Repeating Unit representation for example. For finite molecules, the two most common representations are the SMILES file format, which is proprietary and comes with several conflicting implementations, or the IUPAC's International Chemical Identifier (InChi) key, which is open-source and more standardized, but less human-readable. Both include additional chemical information as well.