Variational Bounds in Positron-Atom Scattering

We have previously applied Dalgarno and Lynn's complete first-order adiabatic correlation function to the problem of low-energy elastic positron scattering by hydrogen and helium atoms. This approach is now extended to yield rigorous lower bounds on the scattering phase shifts in the case of hydrogen and "quasibounds" for helium. In addition, the positron annihilation rate in helium is re-evaluated, and lower values are now found, with the enhancement factor over the Dirac rate varying between 1.5 and 1.8. Some comparisons are made between the present method and other recent work.