- Generally a benzenoid hydrocarbon represented by a Kekulan molecular graph can be partitioned into base sets of Kekulan substructures (the molecules-in-molecule approach). For example, a Kekul structure is a composite of ethylene units (double bonds); and a Clar structure, of ethylene and benzene units. Such a Kekulan substructure accommodates or contains a fraction of the -electrons. We define two kinds of indices; a set of local indices representing the distribution of the -electrons in the whole molecule; and a net index estimating the -electron capacity of the associated partitioning. The stability afforded by such a partitioning increases as the net capacity index becomes larger. Copyright 2005 Taylor & Francis Inc.