Relevant also to this similarity-property principle is the notion of “pharmacophore”: a spatial arrangement of few atoms (or chemical groups) necessary for a drug to be recognized by a biological receptor). Once a pharmacophore is identi-ed, this concept of similarity played, for years, a key role in drug design in the search for new leads that possess the right atoms in the right geometry. With the boom of large chemical databases, the emergence of combinatorial chemistry and high throughput virtual screening, the notions of similarity between compounds and diversity between populations, largely gained importance for organizing and retrieving chemical information. Numerous indices allowing for quantifying similarity, and for example, ranking between hits in querying a database, were dened. Our concern here being QSARs, the reader can be referred for more details on these aspects to specialized articles and textbooks [461-465] and to a recent review [46 and references therein].