On the definition and calculation of the molecular descriptor R′/R

The molecular descriptor R′/R was recently introduced by Randić in this journal [M. Randić, Chem. Phys. Lett. 211 (1993) 478]. The original definition of R′/R can be used for acyclic structures only. The definition is extended here in a way so as to be applicable to any connected structure. The explicit formula for calculation of the R′/R index is derived. The formula provides an easy method to calculate the R′/R index for any connected structure. The advantages of the formula over the algorithm proposed by Randić are pointed out.