On the construction of basis functions for quantum defect theory in arbitrary fields

A numerical method is described for the evaluation of the regular and irregular basis functions used in quantum defect theory for arbitrary potentials (generalized quantum defect theory). The method is related to Milne's phase-amplitude method and yields phase and amplitude parameters which vary smoothly with energy and radius. It also permits the evaluation of these parameters for strongly closed channels, in the absence of a classically accessible radial range.