Configuration-interaction calculations with infinite angular expansions

A simplification of the modified configuration-interaction (MCI) method [S. P. Goldman, Phys. Rev. A 52, 3718 (1995)] is introduced. in its original form, the MCI method improves dramatically the convergence of standard Cl calculations by a modification of the radial representation and an a priori mixing of a large number of angular configurations. In this work, the large but finite number of angular configurations mixed is replaced by an infinite number of configurations. Angular integrations are performed in closed form, resulting in simpler angular momentum calculations. Numerical accuracy and stability are substantially improved as one avoids handling very large values of angular momentum quantum numbers. As a result, calculations involving very high angular mixing can be done with a much shorter amount of CPU time. This angular method can be used in the framework of CI or MCI (i.e., using standard or ordered radial electron coordinates).