Attempts to discover molecular chromophores with large hyperpolarizabilities have largely focused on closed-shell organic species. This contribution explores the use of sum over states (SOS) perturbation theory and the computationally efficient intermediate neglect of differential overlap (INDO) model Hamiltonian, combined with single configuration interaction (SCI) techniques, to calculatic frequency-dependent quadratic molecular hyperpolarizabilities of both coordination complexes and organic radical ion chromophore aggregates having open-shell doublet ground state electronic structures. The correct evaluation of the lowest-lying excited states has been tested by comparison with experimental optical spectra. In the cases considered here, the INDO/SCI-SOS method provides both a good description of linear optical transitions and an adequate prediction of second-order nonlinearity. It is seen that the larger second-order nonlinearities of systems having open-shell electronic states, as compared to those having analogous closed-shell structures, are a consequence of accessible lower-lying charge-transfer excited electronic states. The INDO/SCI-SOS model is attractive for designing new, highly efficient open-shell second-order nonlinear optical chromophores.
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