We use the wave function from the Schwinger multichannel method to calculate ${\mathit{Z}}_{\mathrm{eff}}$—an annihilation parameter of positron-atom (or molecule) scattering. These ${\mathit{Z}}_{\mathrm{eff}}$ are evaluated from the (N+1)-particle wave function and are calculated in a fully ab initio way (in a quantum chemistry sense). Our first applications are for positron scattering against He atoms and ${\mathrm{H}}_2$ molecules. Our results for both systems are in excellent agreement with experimental data at thermal energies.

• Received 29 November 1993

DOI:https://doi.org/10.1103/PhysRevA.49.R1527