Convergence of the projected optical potential in a complex Kohn calculation of elastic e-He scattering

The complex Kohn method, using correlated target states and a projected optical potential, is applied to elastic s-wave scattering of electrons from atomic helium in the energy range of the (1s2s2)2S Feshbach resonance. A projected optical potential is a very useful construct in the study of electron-molecule scattering, but the convergence of this approximation has not been systematically investigated. In this study, the projected optical potential approximation is shown to be convergent and yields an energy and width of the He 2S resonance of 19.357 eV and 14 meV, respectively, in excellent agreement with experiment. (Author)