Molecular systems direct integration

The derivations of the closure relations (2.1.31) and (2.1.33) are both approximate, but we include them in this section in order to complete the discussion of theoretical approaches to atomic fluids. These methods can be applied to molecular fluids directly without fundamental changes (as we will see in Section III), but integral equation approaches developed particularly for molecular systems often involve closure relations that are generalizations and extensions of the ideas presented here. 

---