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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 lless than or equal to18 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 kgreater than or equal to0.1a(0)(-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.