2Institute for Research in Fundamental Sciences (IPM), I. R. Iran
The Wiener polarity index Wp(G) of a molecular graph G of order n is the number of unordered pairs of vertices u, v of G such that the distance d(u,v) between u and v is 3. In an earlier paper, some extremal properties of this graph invariant in the class of catacondensed hexagonal systems and fullerene graphs were investigated. In this paper, some new bounds for this graph invariant are presented. A relationship between Wiener and Wiener polarity index of some classes of graphs are also presented.