  Microscopic cascading of second-order nonlinearities between two molecules
has been proposed to yield an enhanced third-order molecular nonlinear-optical
response. In this contribution, we investigate the two-molecule cascaded second
hyperpolarizability and show that it will never exceed the fundamental limit of
a single molecule with the same number of electrons as the two-molecule system.
We show the apparent divergence behavior of the cascading contribution to the
second hyperpolarizability vanishes when properly taking into account the
intermolecular interactions. Although cascading can never lead to a larger
nonlinear-optical response than a single molecule, it provides alternative
molecular design configurations for creating materials with large third-order
susceptibilities that may be difficult to design into a single molecule.
