A polarized core approximation for atomic structure calculations

The wavefunctions for electron collisions with an N-electron ion may be expanded in terms of functions for the ion core multiplied by orbital functions for the colliding electron. In the expansion one may include both unperturbed ion states, and states of the ion in an electric field, calculated using first-order perturbation theory (the so-called 'pseudo-states'). The radial functions for the colliding electron satisfy a system of coupled integro-differential equations. These equations may be solved at energies which give bound states for the (N+1)-electron system. This method has been used to calculate energy levels for He with inclusion of the unperturbed and the polarized He + ground state. Results for the levels n 1 S, 3 S, 1 P, 3 P, 1 D and 3 D, up to n=3, are much superior to those obtained in the 'frozen core' approximation.