Second hyperpolarizabilities (γ) of open-shell singlet one-dimensional systems: Intersite interaction effects on the average diradical character and size dependences of γ

Significant enhancement of the second hyperpolarizability per unit, γ/n, (a) and its remarkable chain-length dependence (b) are observed for one-dimensional multi-radical systems, i.e., hydrogen chain (H2n) models, in the small average diradical character (yav) region.