The longitudinal static second hyperpolarizability (gamma) has been investigated for models (H-3(+) and H-5(+)) open-shell singlet one-dimensional systems bearing a charged defect. Using various ab initio molecular orbital and density functional theory methods, the values of these cationic symmetric hydrogen chain models having different bond-length alternations (BLAs) are analyzed as a function of the average diradical character as well as by adopting the summation-over-states scheme. These systems exhibit negative gamma values, of which the amplitudes increase nonlinearly with the average diradical character while the chain-length dependence is enhanced by increasing the BLAs. This dependence of gamma with the diradical character is mostly dictated by the strong decrease of the excitation energies. (C) 2007 Elsevier B.V. All rights reserved.