The R-matrix with pseudo-states method: Theory and applications to electron scattering and photoionization

Until a few years ago, calculations for electron scattering from atoms and ions were typically performed using nonperturbative close-coupling-type approximations for low impact energies and perturbative Born-type methods in the high-energy regime. Especially for neutral as well as singly and even doubly ionized targets, however, neither one of those methods is particularly reliable for the "intermediate energy regime", i.e., impact energies near and up to about five times the ionization threshold. This gap has recently been closed by several modifications to the standard close-coupling (R-matrix) formalism. In this paper, the principal ideas behind the R-matrix with pseudo-states (RMPS) method are introduced, and the similarities as well as the differences to related approaches such as the convergent close-coupling (CCC) and the intermediate-energy R-matrix (TERM) methods are discussed. Furthermore, example cases illustrating the need for using such sophisticated approaches are presented. (C) 1998 Elsevier Science B.V.