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Manuscript Title: Calculation of the renormalised electron scattering matrix of a molecule adsorbed on a crystal surface.
Authors: D.K. Saldin, J.B. Pendry
Program title: TAUMOL
Catalogue identifier: AAXA_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 46(1987)129
Programming language: Fortran.
Computer: CRAY-1S.
Operating system: COS.
RAM: 357K words
Word size: 64
Keywords: Solid state physics, Electron diffraction, Surface structure, Renormalised scattering, Factor, Diffuse low energy.
Classification: 7.2.

Nature of problem:
The purpose of this program is to evaluate a quantity required in the calculation of diffuse low energy electron diffraction (LEED) intensities from disordered adsorbates on crystal surfaces. Pendry and Saldin pointed out that such a calculation can be split into three steps: in the first all the scattering paths of the electron prior to its first encounter with the adsorbate are evaluated; in the second the paths between the electron's first encounter and its last with the adsorbate are calculated and finally those paths by which the electron leaves the adsorbate for the last time and makes its way to the detector are traced out. A computer program (DLEED) to perform Steps 1 and 3 above has been developed by a modification of conventional layer- type LEED programs. Under circumstances in which Step 2 may be omitted altogether, the above program may be used on its own. However, especially in the case of molecular adsorbates , Step 2 is expected to be an important part of the calculation. The present program (TAUMOL) performs this part of the computation. Output from TAUMOL, which consists of elements of real and imaginary parts of the renormalised scattering matrices of the adsorbate in the presence of its substrate can be used as input to the DLEED program, which prints out the diffuse LEED intensities so calculated.

Solution method:
The effective (or renormalised) scattering matrix of the adsorbate in the presence of the substrate is related to the bare scattering matrix of the isolated adsorbate via the reflection matrix of a cluster of atoms surrounding the adsorbate. All matrices are represented in an angular momentum basis and the calculation is closely analogous to that of x-ray adsorption near-edge structure (XANES).

Running time:
The test run took 18 s on a CRAY-1S.