In the development of RPA, we used expansions of the first two terms (single and double factors of f ), without considering the higher-order terms. This might normally be a reasonable approximation, as these higher-order terms are only non-zero when three interaction pairs are simultaneously occurring. For interaction sites on a polymer, however, this may be a possibility. To show how this may arise, we consider the interactions relevant to Qin and Larson, where they specifically model polyelectrolyte/salt binding. We first consider only the interaction between a polycation and the surrounding salt species. If the polycation has a high linear charge density, then it is relatively common to observe the following physical arrangement: