Masayoahi NAKANO* (*Osaka Univ.), Hidemi NAGAO and Kizashi YAMAGUCHI*
[Phys. Rev. A 55, 1503 (1997)]
A method for density analysis of static polarizabilities (alpha) and second hyperpolarizabilities (gamma) on the basis of the finite-field (FF) many-electron wavepackets (MEWP) method is developed and applied to evaluation of the longitudinal (alpha) and (gamma) in the dissociation process for a one-dimensional H2 model. Remarkable increases in (alpha) and (gamma) are observed in the intermediate dissociation region. The internuclea distance where the (gamma) is maximized is also found to be larger than that where the (alpha) is maximized. In order to elucidate the characteristics of (alpha) and (gamma) in the dissociation process, we extract their classical pictures describing displacements of two-electron configurations by using (hyper)polarizability densities on the two-electron coordinate plane. It is suggested from these classical pictures that the polarization in the ionic structure contributes primarily to the enhancement of (hyper)polarizability in the intermediate dissociation region, while the polarization in the diradical structure causes the decrease of the (hyper)polarizability at a large internuclear distance. This implies that the experimental search for species with chemical bonds in the intermediate correlation regime is important and interesting in relation to the molecular design of nonlinear optical materials.