Vindicating the Pauling-bond-order concept

The Pauling-bond-order, in which the ith Kekulé structure has weight wi=1; i=1,2,…,K, is modified aswi=1+λfi;i=1,2,…,Kwith fi the number of Fries-type hexagons, andwi=1+λgi;i=1,2,…,Kwith gi=1 if the Kekulé structure is compatible with at least one Clar formula, and gi=0 otherwise. Improvements in reproducing experimental carbon–carbon bond lengths are insignificant.