“…At the same time, multi-electron systems with arbitrary interactions yield the same universal results as one-electron systems 37 , suggesting that the single electron models are suitable vehicles for studying the relationships of graph geometry and topology to the scaling properties of the nonlinear optical tensors. The single electron quantum graph is a well-studied, exactly solvable model of quantum chaos 38,39,40,41,42,43,44,45 . With this in mind, we initiated our studies of the elementary QG model for nonlinear optics by focusing first on undressed edges and calculated the off-resonance first (β ijk ) and second (γ ijkl ) hyperpolarizability tensors (normalized to their maximum values) of elementary graphical structures, such as wires, closed loops, and star vertices 34,46 and to investigate the relationship between the topology and geometry of a graph and its nonlinear optical response through its hyperpolarizability tensors 35 .…”