Dual Chain Perturbation theory: A new equation of state for polyatomic   molecules
  In the development of equations of state for polyatomic molecules, thermodynamic perturbation theory (TPT) is widely used to calculate the change in free energy due to chain formation. TPT is a simplification of a more general and exact multi-density cluster expansion for associating fluids. In TPT all contributions to the cluster expansion which contain chain - chain interactions are neglected. That is, all inter-chain interactions are treated at the reference fluid level. This allows for the summation of the cluster theory in terms of reference system correlation functions only. While highly successful, TPT has many handicaps which result from the neglect of chain - chain interactions. The subject of this document is to move beyond the limitations of TPT, and included chain - chain contributions to the equation of state. 
Bennett D. Marshall
cond-mat.soft