Complexity: The complexity rating of the compounds is a rough estimate of how complicated a structure is, seen from both the point of view of the elements contained in it and its structural features including symmetry. However, neither stereochemistry nor isotope labeling are used as auxiliary criteria. The value is computed using the Bertz/Hendrickson/Ihlenfeldt formula (see W. D. Ihlenfeldt, Ph.D. Thesis, TU Munich, Germany, 1991). A scaling factor for aromaticity is used so that the complexity of benzene is the same as that of cyclohexane. It is a floating point value, ranging from 0 (simple ions) to several thousand (complex natural products). Generally, larger compounds are more complex than smaller ones, but highly symmetrical compounds, or compounds with few distinct atom types or elements are downgraded. Complexity is only loosely correlated with synthetic accessibility.