Randić's molecular connectivity index

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

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