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Manuscript Title: Calculation of low energy atomic diffraction (LEAD) intensities.
Authors: R.J. Blake
Program title: ATSCAT
Catalogue identifier: ACCX_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 33(1984)425
Programming language: Fortran.
Computer: NAS AS7000.
Operating system: MVT.
RAM: 494K words
Word size: 64
Keywords: Theoretical methods, Solid state physics, Low energy atom, Diffraction intensities, Corrugated soft wall Potentials, Stabiliser, Coupled channels, Bayliss-peel algorithm, Error control, Experiment.
Classification: 7.4.

Nature of problem:
Low energy atom scattering experiments (E < 200 meV) on crystal surfaces provide diffraction patterns which reflect the two dimensional periodicity of the surface and the nature of the surface-atom interaction potential. This data is compared with theoretically produced intensities in order to determine unknown surface structures or, for known surface structures, to test the validity of ab initio atom-surface interaction potentials.

Solution method:
The wavefunction for the atom in the periodic interaction potential V(r) is expanded in terms of a set of basis functions described by the symmetry of the surface. This basis set includes both open and closed channels. A set of close coupled differential equations dependent upon the coordinate normal to the surface results. Linearly independent solutions of these equations are propagated from a point within the classical turning surface of V(r) out to the asymptotic potential-free- region, using the highly accurate Bayliss Peel algorithm. A new form of error control and a strong stabilising package are used.

The interaction potential is assumed to be real and to have a centre of inversion. The program is dimensioned for up to 60 channels and up to 29 Fourier coefficients of the potential.

Running time:
Very problem dependent, the times taken by the test problems 1 and 2 were 22 s and 56 s respectively.