Configuration-interaction approach for high-lying singly and doubly excited states of two-electron systems

We introduce a configuration-interaction expansion in terms of Sturmian functions, which provides very accurate nonrelativistic energies for the high-lying singly and doubly excited states of a two-electron system. The approach, which requires a substantially smaller basis size by contrast to the standard configuration interaction methods, provides accurate results even for large total angular momenta. Key features are the use of different numbers of Sturmian functions for each electron, the inclusion of many different pairs of nonlinear parameters in the expansion, as well as complex scaling. The accuracy and convergence of the method, applied to helium, increases with the degree of excitation of one of the two electrons.