Inclusion of contact dissimilarity

To allow for interactions, the mean intermolecular energy E is introduced as follows  

The configuration integral on the right hand side of equation (3.66) ensures that E is the average energy of the entire system. It follows that the partition function for the system is given by 

Note that Flory chose, somewhat arbitrarily, to employ a van der Waals interactional energy. This sets the attraction of molecules in a dense fluid as being inversely related to the particle volume ( oc 1/r ). 

---