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Manuscript Title: Calculation of the Green's function for a crystal surface or interface.
Authors: F. Maca, M. Scheffler
Catalogue identifier: AADF_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 38(1985)403
Programming language: Fortran.
Computer: TELEFUNKEN 440.
Operating system: BS3.
RAM: 80K words
Word size: 48
Keywords: Solid state physics, Green's function, Crystal field, Interface, Adsorbate, Density of states, Charge density, Layer kkr method, Scattering multiple, Condensed matter.
Classification: 7.3.

Nature of problem:
The computer code described in this paper allows the calculation of the Green's function of a three-dimensional system with two-dimensional translational symmetry. Examples for its application are the crystal surface and the interface between two crystals. The Green's function contains detailed information about the electronic structure. It directly allows to evaluate the surface electron charge density and the local and projected density of states. It is also a key factor determining the photoemission current.

Solution method:
The method is a two-dimensional analog of the Korringa Kohn Rostoker (KKR) approach. The crystal is divided into planar layers parallel to the surface and the potential of these layers is treated in the muffin- tin approximation. The atomic muffin-tin potentials and the geometric structure are the required input parameters. The Green's function is evaluated in a spherical waves expansion, with basis functions centered at any atom of interest.

The semi-infinite crystal must be composed of identical planar layers parallel to the surface. If there is an overlayer, its two-dimensional periodicity must be a coincidence structure of the substrate layers. The unit cell of any layer may be composed of one, two or four different muffin-tin potentials. For the potential barrier between the top atomic layer and vaccum we assume a simple step function; the generalization to more complicated barrier requires the replacement of one subroutine.

Running time:
The time for the test run i.e. a c(2*2) adsorbate layer on the (100) surface of a fcc crystal is 200 s on a Telefunken 440 (one energy point only). 23 plane waves and 3 phase shifts per atom are used and the Green's function is evaluated for the adsorbate and the first substrate layer. The most time consuming parts of the program are: a) The calculation of the substrate reflection matrix by the layer- doubling scheme and b) the transformation of the Green's function matrix from the plane- wave to the spherical-wave basis.