Basic concepts of the R-matrix method and relationship to other theories

A review of the basic concepts of the R-matrix method and its relationship to other theoretical approaches used for electron atom and electron molecule collisions is given. The division of configuration space into an internal and an external region is described. It is shown that the R-matrix approach corresponds to a configuration interaction expansion of the total wave function in the internal region. Various methods of representing the continuum orbital basis are then evaluated. Finally it is shown that the R-matrix expansion is formally equivalent to the close coupling plus correlation term method, but the form of the expansion makes the application of standard bound state computer codes particularly straightforward. (Auth.)