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Manuscript Title: COULN: a program for evaluating negative energy Coulomb functions.
Authors: C.J. Noble, I.J. Thompson
Program title: COULN
Catalogue identifier: ACCU_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 33(1984)413
Programming language: Fortran.
Computer: NAS 7000.
Operating system: MVT.
RAM: 650K words
Word size: 8
Keywords: General purpose, Coulomb, Whittaker, Continued fraction, Pade, Asymptotic, Molecular physics, Theoretical methods.
Classification: 4.5.

Nature of problem:
Program COULN calculates exponentially decaying Whittaker functions, Wkappa, mu(z)corresponding to negative energy Coulomb functions. The method employed is most appropriate for parameter ranges which commonly occur in atomic and molecular asymptotic scattering problems using a close-coupling approximation in the presence of closed channels.

Solution method:
The asymptotic series for the Whittaker function is analytically continued using a continued fraction representation devised by Nesbet. Functions corresponding to higher orbital angular momenta are computed by recursion.

Restrictions:
Program COULN assumes real negative energies, E = - |k|**2, non-negative real values of orbital angular momenta, l (integer), and separations, r. The charge product, Z, must be non-zero. The method becomes less efficient for large values of |Z| and small values of the product Z = |k|r. It is suitable for z/Z greater than 0.5 on IBM computers and greater than 0.1 on the CRAY-1S in double precision.

Running time:
The test run required 5.2 s on the NAS 7000.