Internal consistency in the close-coupling approach to positron collisions with atoms

The positron-atom scattering problem contains the rearrangement channel of positronium (Ps) formation. While this makes the problem particularly difficult to calculate, it has the unusual benefit of validation via consideration of the internal consistency of the vastly different one- and two-centre close-coupling approaches. For example, the ionisation cross section in the former must be the same as the sum of breakup and Ps formation cross sections in the latter. This places a severe test on both approaches, which we review here for positron scattering on hydrogen and helium atoms