We assume normalized coefficients henceforth, in which case the numerator in this expression equals unity, as in Eq. (4.18). If p i = 1/n, indicating equal probabilities at each site, then Eq. (4.18) affords PR = n. In general, the PR may be interpreted as the number of sites over which the wave function delocalizes, and for that reason it has sometimes been called a collectivity index. The presence of a reciprocal in Eq. (4.18) seems to have led to some confusion, whereby this quantity is sometimes called the inverse participation ratio (IPR). However, calling the quantity defined in Eq. (4.18) a PR is consistent with the earliest examples in the literature, and perhaps more importantly it means that the PR increases (rather than decreases) as more and more monomers participate in the excitation. With Eq. (4.18) taken to define the PR, then its inverse is