The 1/6 is a combinatoric factor that is determined from the symmetry of an unlabeled diagram, accounting for the number of ways in which the diagram can be rearranged yet retain the same topology. In the second version of the above expression, we use the assumption that we will make in the rest of this paper, in that we will assume a homogeneous phase where the density ρ α (r) = ρ α = ρ α is given by its average value. This diagrammatic representation of the excess free energy density will be the starting point of all coacervate models, as it isolates the theoretically challenging aspect of the model. The ideal mixing entropy common to essentially all coacervation models is given as: