(a),(b) Schematic illustrations of one of many possible configurations of the three-body cluster at low and high temperature, respectively. At low , only a few configurations with small energy contribute to a given virial coefficient . At high temperature, many configurations with small and large energy contribute. (c) Shows the cluster expansion parameter, or fugacity, )], where denotes the chemical potential and the Boltzmann constant, as a function of for the example of infinitely strong interactions. The thermodynamic potential , which determines the energy and entropy, can be written as , where is determined by the energy spectrum of the th cluster. The contribution of the th cluster is suppressed by a factor of compared to that of the ()th cluster. The suppression is very effective for (i.e., only terms with and are needed), but higher-order clusters are needed for .