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Manuscript Title: MRVAC: a variational correction method for solving differential equations with r**-n coupling.
Authors: M.R.H. Rudge
Program title: MRVAC
Catalogue identifier: ACCV_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 34(1984)187
Programming language: Fortran.
Computer: VAX 11/750.
RAM: 100K words
Word size: 3
Keywords: General purpose, Differential equation, Asymptotic solutions, Scattering.
Classification: 4.3.

Nature of problem:
A wide variety of scattering problems can be reduced to solving sets of coupled differential equations wherein the coupling can be represented as a matrix series in r**-n at large values of r. The program is designed to produce solutions of the equations at an input value r = Ro, and to calculate a K-matrix to a specified accuracy from an input R-matrix at r = Ro.

Solution method:
A variational correction technique described by Rudge is used in the program.

The code is written for completeness to include the Coulomb case (an r-1 coupling) but the current version handles this case much less efficiently than the non-Coulomb cases for which it is primarily designed. An accuracy check is a feature of the program but if this is not met within a certain number of iterations then there is a cut-out in the program which flags cases where this occurs.

Running time:
A twenty channel problem takes about two minutes on the VAX 11/750 mini- computer.