Coupled channel equation for potentials with a Coulomb singularity

A method for solving the coupled channel equation for potentials with a Coulomb singularity is presented: At small r, where the Coulomb term is dominant, the solution is expressed by using the variation of constants method in terms of the Coulomb functions F_l and G_l. At large r, where the Coulomb potential is no longer important, one returns to the usual variable phase method which expresses the solution in terms of Bessel functions and Neumann functions. Furthermore, the use of an interpolation scheme for the energy dependence considerably reduces the amount of computation. The advantages of this approach when used in conjunction with the point group symmetry are illustrated by using a realistic potential taken from a full potential linearized augmented plane wave (FLAPW) calculation for Cu.