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Manuscript Title: A new no-exchange R-matrix program.
Authors: V.M. Burke, P.G. Burke, N.S. Scott
Program title: RMATRX NX
Catalogue identifier: ACGP_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 69(1992)76
Programming language: Fortran.
Computer: CRAY X-MP/416.
Operating system: VMS.
RAM: 670K words
Word size: 64
Peripherals: disc.
Keywords: Atomic physics, Electron-atom scattering, Electron-ion, Scattering, R-matrix, No-exchange, Radial integrals, Angular integrals, Hamiltonian matrix, Ls coupling, Buttle correction, Preprocess.
Classification: 2.4.

Nature of problem:
Electron-atom and electron-ion Hamiltonian matrix eigenvalues and eigenvectors are calculated in LS coupling neglecting exchange. This gives accurate results for high partial waves which can be used to supplement low partial wave results including exchange.

Solution method:
The R-matrix method is used implementing a new faster technique for calculating the elements of the Hamiltonian matrix when exchange and the quadratically integrable functions are omitted. A file is written intended for input to a program which continues the calculation into the asymptotic region.

The exchange operator and the quadratically integrable functions are omitted. The calculations are carried out using the non-relativistic Hamiltonian in LS coupling.

Unusual features:
The program RMATRX NX is designed in three stages. NXANG calculates all the angular integrals occurring in the Hamiltonian matrix for the system for a given range of partial waves, NXRAD calculates all the corresponding radial integrals and NXHAM forms each Hamiltonian matrix, diagonalises it and writes an output file for an asymptotic program. All the dimensions in the program are contained in parameter statements and can be set by a preprocessor PNX, also described in this paper. The data for RMATRX NX (and for PNX) can be considerably simplified when running NX as a continuation of RMATRX, adapted as explained in this paper.

Running time:
The RMATRX NX code is considerably faster than RMATRX, the time-saving depending on the problem. In cases where RMATRX STG2 is the most time- consuming section of the program an increase of speed of the order of 10 to 100 is found. The program becomes faster as total angular momentum increases. The test run took 70 seconds.