Randić's molecular connectivity index

The Randić index, also known as the connectivity index, of a graph is the sum of bond contributions ${\displaystyle 1/(d_{i}d_{j})^{1/2}}$ where ${\displaystyle d_{i}}$ and ${\displaystyle d_{j}}$ are the degrees of the vertices making bond i ~ j.

History

This graph invariant was introduced by Milan Randić in 1975.^[1] It is often used in chemoinformatics for investigations of organic compounds.