Variational calculation of positron-atom scattering using configuration-interaction-type wave functions

The Kohn variational method is used with a configuration-interaction (CI)-type wave function to determine the phase shifts and Z_eff for positron-copper scattering. The method is first tested for positron-hydrogen scattering and it is found to give phase shifts and Z_eff within 1–2% of the best previous calculations. Although the phase shift for Cu converged more slowly with L_max (the maximum angular momentum of the electron and positron orbitals included in the short-range basis), it was still possible to get reliable estimates of the phase shifts by including orbitals with l<~18 and the use of an extrapolation technique. Calculation of Z_eff was more problematic since the convergence of Z_eff with respect to L_max was very slow. Despite the uncertainties, it was clear that the p-wave phase shift was showing signs of forming a shape resonance at about 0.5 eV incident energy. This resulted in a p-wave contribution to Z_eff that was larger than that of the s wave for k>~0.1a₀^-1. Speculative calculations based upon a model potential suggest that a p-wave shape resonance centered at thermal energies, e.g., about 0.025 eV, could result in a thermally averaged Z_eff exceeding 10 000.