An important constraint we leave implicit is that the pair s, s cannot be the same as s , s , which we will include explicitly in a more compact notation. This notation will leave implicit the spatial dependence of the functions f , so that we only need to specify the species α and γ and the monomer indices s, s ; so we replace f α,γ (r i,α (s), r j,γ (s )) → f α,γ|s,s . We can show how this is incorporated into diagrams for the excess free energy expansion given in Equation . Let us consider a simple pairwise diagram for a system with only polyelectrolytes of length