The variational method in atomic scattering

This paper reviews the application of the variational method to the scattering (both elastic and inelastic) of electrons and positrons by atoms. The coupled equations of scattering theory are formulated using a pseudostate expansion. The Kohn variational procedure for multichannel scattering is derived. Many variants are considered, including the inverse Kohn procedure, the ''optimized minimum norm'', the ''optimized anomaly free'', and the ''variational least squares''. Applications to both model and realistic problems are described. Results obtained for scattering by hydrogen, and by complex atoms are discussed. (Auth.)