As with the rotatable-bond count, Kier treats flexibility as a structural attribute that can be derived directly from the molecular graph . Kier's reference point for a perfectly flexible molecule is the infinite chain of carbon atoms with sp 3 hybridization (Csp 3 ), which marks the point where the flexibility index φ is defined to be infinite. The definition of φ moves on to quantify the extent to which structural features (like having a finite number of atoms, branching, cycles, and the presence of heteroatoms) decrease this perfect flexibility . Kier defines φ using two of the κ shape indices he had previously introduced, 1 κ[16] and 2 κ [17]. 2 κ accounts for the number of atoms and relative cyclicity by counting all 2-bond fragments in a molecular graph , whereas 1 κ is the 1-bond fragment count and hence encodes the branching . An additional factor, α, is introduced to account for the contributions of atom types other than Csp 3 to the shape . The normalized shape indices 1 κ α and 2 κ α are combined and scaled by the number of atoms A, yielding the Kier φ index to describe the overall molecular flexibility: