CLOSE-COUPLING FORMALISM FOR ELECTRON POSITRON-ATOM SYSTEMS AND UNDERLYING SCATTERING-THEORY/

The close-coupling method for electron/positron-atom scattering promis es to give a complete description of the scattering process provided t he space of target states is properly spanned. This paper will discuss the structure of the equations, emphasising questions of stability as sociated with the expansions over sets of target states. For electron- atom scattering, the character of the solution is discussed and a simp le example is given to illustrate the non-convergence of the half-shel l T-matrix. This lack of convergence can be fixed by application of th e symmetrisation boundary condition, leading to new forms of the equat ions. For positron-atom scattering, the standard equations yield conve rgent half-shell T-matrices for all but the largest calculations.