The Schwinger multichannel method (SMC) calculations for Zeff were off by a factor of Z

Abstract

Our previously reported Z eff values were in accidental agreement with experiment. They were actually underestimated by a factor of Z. In order to full understand the nature of the discrepancies with experimental data, we carried out several tests for positron–He collisions. We first tried many different (Cartesian Gaussian) basis sets in a single-excitation configuration–interaction (CI) framework. Then, we included single- and double-excitations of the target in the CI expansion (CISD) of the scattering wave function. Finally, we tried to mimic the virtual positronium cusp. It seems that cusp description should indeed be further improved.