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

We investigate the longitudinal static second hyperpolarizabilities (gamma) of open-shell singlet one-dimensional systems. The intersite interaction effects on the average diradical character and size dependences of gamma are examined using hydrogen chain models (H-2n) with different chain lengths and bond-length alternations (BLA) by the ab initio MO full-Cl and coupled cluster methods. These systems exhibit significant enhancement of gamma/n in the small average diradical character region compared to diradical H-2. The BLA is found to reduce the maximum gamma/n and to move the diradical character giving the maximum gamma/n to intermediate region. These systems also show remarkable chain-length dependences of gamma/n (comparable to those of closed-shell systems) in the small average diradical character region. (c) 2006 Elsevier B.V. All rights reserved.