A number of elaborate scoring systems has also been devised to express topological and total molecular complexity. However, arguably, the most popular general scoring system for molecular complexity has been introduced by Bertz, considering both the topological features of a molecular skeleton through the C(η) score calculated for subgraphs of a molecular graph, and its elemental content via the C(E) component accounting for the presence of heteroatoms. The Bertz total molecular complexity index CT is then simply calculated through summation of the topological and the elemental diversity scores, where CT = C(η) + C(E). The Bertz complexity index is used as a molecular complexity measure for compound evaluation by Pubchem, a popular public repository of chemical information. Another exemplary complexity scoring system based on connectivity of subgraphs was proposed by Bonchev resulting in two complexity indices TC and TC1. Complexity measures based on the analysis of substructures include for instance the empirical and intuitive system devised by Whitlock which takes into consideration the number of alicyclic rings, non-aromatic unsaturations, heteroatoms and stereogenic centres. Here the complexity score S is a sum of the appropriately weighted numbers of given components with no regard for the skeletal connectivity. A very similar approach and modification of Whitlock's complexity system was presented by Barone and Chanon, who in their scoring index additionally account for the number of substituents and ring sizes. Complexity metrics based on other features include the score of Allu and Oprea relying on atomic electronegativities and bond parameters or the metric of von Korff and Sander calculating a fractal dimension of a molecule. Complexity focusing on the molecular shape can also be described by the Kappa shape indices through a comparison of the analysed molecule to the extreme shapes for a structure with the same number of atoms. The Kappa indices are calculated in respect to a fully linear and fully connected structure, as well as, to a star shape scaffold. The disadvantage of the Kappa scoring system is the necessity for comparison of different indices which can be inconvenient for the analysis of larger molecular libraries.