The Effects of Geometry on the Hyperpolarizability
  Extensive studies in the past have focused on precise calculations of the nonlinear-optical susceptibility of thousands of molecules. In this work, we use the broader approach of considering how geometry and symmetry alone play a role. We investigate the nonlinear optical response of potential energy functions that are given by a superposition of force centers (representing the nuclear charges) that lie in various planar geometrical arrangements. We find that for certain specific geometries, such as an octupolar-like molecule with donors and acceptors of varying strengths at the branches, the hyperpolarizability is near the {\em fundamental limit}. In these cases, the molecule is observed to be well approximated by a three-level model - consistent with the three-level ansatz previously used to calculate the {\em fundamental limits}. However, when the hyperpolarizability is below the {\em apparent limit} (about a factor of thirty below the {\em fundamental limit}) the system is no longer representable by a three-level model; where both two-level and a many-state models are found to be appropriate, depending on the symmetry. 
Mark G. Kuzyk and David S. Watkins