Full configuration interaction calculations of the second hyperpolarizabilities of the H4 model compound: Summation-over-states analysis and interplay with diradical characters

The second hyperpolarizability (gamma) of the one-dimensional H-4 model compound has been calculated at the full configuration interaction level to describe its relationships with the diradical characters y(i) [the occupation numbers of the lowest unoccupied natural orbital (LUNO) + i] and the geometry. It is found that the system with intermediate y(0) (= 0.527) and small y(1) (= 0.178) exhibits the largest. value [enhanced by a factor of 9 compared to that of a nearly closed-shell H-4 analog (y(0) = 0.099, y(1) = 0.029)], where both the outer H-H distances are slightly larger than the inner one. These results provide a molecular design guideline for enhancing and controlling the third-order nonlinear optical properties of singlet multiradical linear molecular systems such as multiradical organic aggregates, supermolecular systems, and extended metal atom chains. (C) 2012 American Institute of Physics. [doi:10.1063/1.3675684]