Many-electron hyperpolarizability density analysis: Application to the dissociation process of one-dimensional H-2

A method for density analysis of static polarizabilities (alpha) and second hyperpolarizabilities (gamma) on the basis of the finite-field (FF) many-electron wave packets (MEWP) method is developed and applied to evaluation of the longitudinal alpha and gamma in the dissociation process for a one-dimensional H-2 model. Remarkable increases in alpha and gamma are observed in the intermediate dissociation region. The internuclear 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.