THE HYPERPOLARIZABILITY OF LI 1S(2)2S S-2 - A TEST-CASE FOR ADVANCED THEORIES OF ELECTRONIC-PROPERTIES

From the existing calculations of the hyperpolarizability, gamma, of L i 1s(2)2s 2S, it is evident that this quantity is very difficult to ob tain reliably. Here we present a state-specific approach for the compu tation of linear and non-linear polarizabilities of polyelectronic gro und or excited states and apply it to the prototypical He 1S and Li 2S States. The problem is formulated as one of searching for the complex eigenvalue of a non-Hermitian matrix, H, constructed from multidimens ional function spaces, Q and P, which are chosen according to the spec tral features and electronic structure of the system under investigati on. For example, in the present case, singly and selected doubly excit ed states are included, representing valence and core-valence electron correlation and field-induced excitations. On the other hand, for rea sons of economy the pair correlation of the K-shell electrons in Li is neglected. Our results, alpha(He) = 1.389 au, gamma(He) = 42.78 au an d alpha(Li) = 164.9 au, gamma(Li) = 3.7 x 10(4) au are compared with p revious ones, obtained from methods using fixed basis sets or r(ij)-de pendent expansions. For gamma(Li), the observed discrepancy suggests t hat additional theoretical and certainly experimental information woul d be very useful.