Scattering of near-zero-energy electrons and positrons by H2

The parameters for S-wave elastic scattering of near-zero-energy electrons and positrons by H-2 molecules are calculated using the stabilization method with explicitly correlated Gaussians. The confined variational method is applied to optimize the Gaussians to describe the short-range interaction of incident e(+/-) with H2 in the fixed-nuclei approximation. For e(+)- H-2 scattering the scattering length of previous work [Phys. Rev. Lett. 103, 223202 ( 2009)] is substantially improved. More importantly, for e(-)-H-2 scattering, from first principles, the scattering length is computed as a function of the internuclear distance. In the case that the two nuclei are at the equilibrium distance the results are in a good agreement with values derived from fitting experimental total and diffusion cross sections to the modified effective range theory.