  A simple method to include the strong force in atom-antiatom scattering is
presented. It is based on the strong-force scatteringn length between the
nucleon and antinucleon. Using this method elastic and annihilation cross
sections are calculated for hydrogen-antihydrogen and helium-antihydrogen
scattering. The results are compared to first-order perturbation theory using a
pseudo potential. The pseudo-potential approach works fairly well for
hydrogen-antihydrogen scattering, but fails for helium-antihydrogen scattering
where strong-force effects are more prominent.
