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Manuscript Title: ROMPIN: a relativistic optical model program for pion-nucleus scattering.
Authors: D.R. Giebink, D.J. Ernst
Program title: ROMPIN
Catalogue identifier: ABBP_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 48(1988)407
Programming language: Fortran.
Computer: VAX11/780.
Operating system: VMS 4.1.
RAM: 1000K words
Word size: 32
Peripherals: disc.
Keywords: Particle physics, Elementary, Relativistic optical Potential, Fermi-average, Pion, Elastic scattering, Single-charge-exchange Scattering, Double-charge-exchange Scattering, Phenomenological model.
Classification: 11.6.

Nature of problem:
Pion-nucleus elastic scattering and single- and double-charge exchange to analog states are computed from a momentum-space optical potential. The optical potential is calculated with an exact performance of the fermi averaging integral and with fully relativistic kinematics by utilizing relativistic three-body recoupling coefficients.

Solution method:
A fermi-averaged, momentum-space optical potential is computed in the impulse approximation using relativistic angular-momentum recoupling coefficients to couple the nuclear wave function to the off-shell pion- nucleon amplitude. The optical potential is determined in an angular- momentum basis and inserted into either the Lippmann-Schwinger or the Klein-Gordon equation. This equation is linearized and solved by matrix inversion. Differential and total scattering cross sections and polarizations are computed. User modifiable routines for the nuclear wave function, the off-shell pion-nucleon transition amplitude, and an optional second-order potential are provided. The wave function is read from an external file, which can be prepared using the program RMPWFN (also described in this article). This program computes the nuclear wave function for spin-0 and spin-1/2 nuclei from harmonic oscillator expansion coefficients in the form required for input to ROMPIN. In ROMPIN the pion-nucleon amplitude is computed in a single subroutine, its off-shell behavior is specified via function statements, and its on- shell behavior is read from an external file. The second-order potential is also computed in a single subroutine and can be modified by the user. Two principal options are provided for the determination of the first-order strong potential. The first computes a fermi-averaged potential. The second computes a potential in the optimally factorized approximation. Each of these principal options includes the following secondary options: (1) retained charge or conserved isospin computation, (2) relativistic potential theory with or without multiplicative Lorentz transformation factors, (3) separable potential or Chew-Low behavior of the off-shell pion-nucleon amplitude, and (4) an optional choice of energies at which the pion-nucleon amplitude is evaluated including the three-body, the mean-spectral-energy, the closure and the impulse approximations. Exact inclusion of the Coulomb potential is also provided as an option. The theory used is quite general and most of the program is written to accommodate projectiles and nuclei with any spin; however, the routines that solve the Lippmann-Schwinger equation, compute the cross sections and wave functions, and part of the routine that computes the fermi-averaged potential are specifically written for reactions that are diagonal in the total, total-orbital, and total-spin angular momenta (i.e., spin-0 + spin-0 and spin-0 + spin 1/2 parity- conserving reactions). The dimensions of large arrays are computed from information supplied in an external parameter file.

Restrictions:
Spin-0 and spin 1/2 nuclei in fermi-averaged calculation. Second-order potential and optimally-factorized calculation for spin-0 nuclei only. No Wigner spin precession. Charge exchange reactions can only be computed using the conserved isospin option and this option does not support the additional Coulomb potential.

Running time:
16O at 163 MeV, optimally factorized calculation: 3 minutes
16O at 163 MeV, fermi averaged computation: 25 minutes
13C at 163 MeV, fermi averaged computation: 35 minutes.