where z is the coordination number, the number of nearest neighbors for a lattice site, each one occupied either by one chain segment or a solvent molecule. That is, x N 2 is the total number of polymer segments (monomers) in the solution, so x N 2 z is the number of nearest-neighbor sites to all the polymer segments. Multiplying by the probability ϕ ϕ 1 that any such site is occupied by a solvent molecule, we obtain the total number of polymer-solvent molecular interactions. An approximation following mean field theory is made by following this procedure, thereby reducing the complex problem of many interactions to a simpler problem of one interaction.