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Manuscript Title: Calculation of elastic diffuse LEED intensities from disordered adsorbates.
Authors: D.K. Saldin, J.B. Pendry
Program title: DLEED
Catalogue identifier: AALY_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 42(1986)399
Programming language: Fortran.
Computer: CRAY-1S.
Operating system: COS.
RAM: 160K words
Word size: 64
Keywords: Solid state physics, Diffuse low energy, Electron diffraction, Surface structure, Disordered adsorbates, Disorder lattice-gas.
Classification: 7.2.

Nature of problem:
To calculate the diffuse intensity distribution produced by low-energy electrons elastically scattered from an adsorbate randomly adsorbed on locally identical sites on an otherwise clean crystal surface, and (optionally) to derive from this a Y-function characteristic of the local bonding geometry of the adsorbate.

Solution method:
In our first paper on diffuse LEED (Pendry and Saldin) we suggested a three-step approach to the calculation of the diffuse intensities: first summed (step 1) are all scattering paths prior to the incident electron's first encounter with the adsorbate. Secondly (step 2) all the multiple scattering processes between the electron's first and after its last interaction with the adsorbate are evaluated and finally (step 3) those paths after the final encounter with the adsorbate. The second step is performed by a cluster program analogous to one employed in the calculation of X-ray absorption near edge structure. This program (TAUMOL) has been developed and will be the subject of a subsequent publication. For atomic adsorbates this step can frequently be omitted to a good approximation. Steps 1 and 3 involve mainly multiple scattering by the substrate, evaluated in a layer geometry as in conventional LEED calculations. This is broadly the strategy we follow and steps 1 and 3 are performed by the present program. The new feature that we suggest and implement here is that step 3 be performed in exactly the same manner as step 1, for a time-reversed exit beam, a procedure justified by the reciprocity theorem.

Restrictions:
At the moment, the substrate is restricted to consist of layers having two-dimensional unit cells containing one atom each.

Running time:
A typical calculation of (24*24) arrays of diffuse intensities and Y- functions for the system O/W(100) for an electron energy of about 50eV. takes about 5 minutes on a CRAY-1S.