Abstract
The variations in the longitudinal second hyperpolarizabilities (γ) in the bond-dissociation processes of molecules (H2 and LiH) are investigated by the finite-field (FF) approach using several ab initio molecular orbital (MO) methods, i.e., spin-restricted methods (RHF, RMPn (n = 2,4), RHF coupled cluster (RCC)) and spin-unrestricted methods (UHF, UMPn (n = 2,4), UHF coupled cluster (UCC)) as well as approximate spin-projected methods (APUHF and APUMPn). The variation of γ in the bond dissociation process is expected to elucidate the feature of third-order nonlinear optical (NLO) properties of molecules, e.g., open-shell organic and transition metal compounds, which belong to the intermediate and strong correlation regime. The calculation methods except for CC exhibit a cusp in the variation of γ at the triplet instability threshold, and remarkable electron-correlation dependency of the variation in γ is observed in the intermediate and large bond-distance region, corresponding to intermediate and strong correlation regime, respectively. From the results of the most accurate methods (UCCSD(T) or RCCSD(T)) employed in this study, not only significant increase and decrease behavior of the magnitude of γ value is observed in the intermediate dissociation region of these molecules but also even the change in the sign of γ value appears in the dissociation process of LiH.