On the origin of the large electron correlation contribution to the hyperpolarizabilities of some diacetylene rare gas compounds.

A comprehensive study of the linear and nonlinear molecular optical properties of HRgC(4)H, where Rg = Ar,Kr,Xe, has been performed. Dynamical electron correlation effects were computed by employing the coupled cluster methodology. A large electron correlation contribution to the nonlinear properties of HArC(4)H has been revealed. This contribution decreases by increasing the atomic number of the inserted rare gas atom. In order to interpret the origin of this noteworthy property, the complete active space self-consistent field method was employed. We have performed a systematic study of the linear and nonlinear electric properties by modifying the active space. The calculations have shown the significant contribution of the doubly excited sigma(*2) configuration and a negligible contribution of pi(*2). A quite remarkable discrepancy between numerically and analytically evaluated hyperpolarizabilities has also been observed for HArC(4)H. This was attributed to the contribution of near degenerate states.