Stochastic variational calculation of zero-energy positron scattering from H, He, and H-2

The confined variational method is used to generate a set of energy-optimized explicitly correlated Gaussians to describe positrons interacting with H, He and the H 2 molecule. These basis functions are then used in a Kohn variational calculation of the scattering lengths and zero-energy annihilation parameters, Z eff. The results for the H and He atoms are consistent with previous high-quality variational calculations while the annihilation parameter for H 2 is consistent with experiment. An extensive tabulation of the H 2 scattering parameters as a function of internuclear distance confirm the existence of a virtual state at R?3.4a 0. The scattering length was -2.71a 0 and Z eff was 15.7 at the mean H 2 internuclear distance of 1.45a 0.