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Manuscript Title: Classical collisions of protons with hydrogen atoms.
Authors: D. Banks, K.S. Barnes, P.E. Hughes, I.C. Percival, D. Richards, N.A. Valentine, J.McB. Wilson
Catalogue identifier: ACXO_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 13(1977)251
Programming language: Fortran.
Computer: CDC 6600.
Operating system: SCOPE 3.4.
RAM: 47K words
Word size: 60
Keywords: Molecular, Classical, Trajectory, Body, Inelastic, Proton, Scattering, Charge, Hydrogen, Monte carlo, Cross section, Ejection, Energy transfer, Representation, Ionization, Exchange, Transfer charge, Impact parameter, Capture, Rearrangement, State, Inverse square, Runge-kutta, Other.
Classification: 16.9.

Nature of problem:
The program solves the equations of motion for the interaction of 3 charged particles, obtaining final states in terms of initial states, and energy transfers, angles of ejection, and final cartesian co- ordinates of relative motion. Using a Monte Carlo method on many orbits total ionization and charge transfer cross sections, integral energy transfer cross sections and moments of energy transfers are estimated. Facilities are provided for obtaining angular distributions, momentum transfer cross sections and for comparisons with various approximate classical theories.

Solution method:
The equations of motion are solved using stepwise fourth-order Runge- kutta integration with automatic steplength change. Selection of initial conditions is determined by the user, usually as a statistical distribution determined by a pseudorandom number subroutine. Classical representation theory and transformation methods are extensively used.

Precise cross sections require considerable running time, particularly at low incident particle energies.

Unusual features:
Facilities are provided for storing essential orbit data on magnetic tape for later analysis, thus avoiding repeated numerical integrations.
The program is structured so that its size can be reduced, as shown in sec. 15.

Running time:
The test run takes approximately 0.3 s per orbit on a CDC 6600 for 50 KeV protons incident on classical hydrogen atoms with 13.6 eV binding energy. For a given precision the time of integration increases approximately inversely with the velocity of the incident particle. An estimate of an ionization cross section at one energy in the neighbourhood of the maximum to within about 10% take less than 2000 orbits. Energy transfer integral cross sections take longer by a factor of about 5.
Running time depends critically on careful use of the program, particularly impact parameter ranges and error parameters. The main references should be consulted before extensive use.