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Manuscript Title: A self-consistent surface Green-function (SSGF) method for the calculation of isolated adsorbate atoms on a semi-infinite crystal.
Authors: J. Bormet, B. Wenzien, M. Scheffler
Program title: fhi93ssgf
Catalogue identifier: ACPV_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 79(1994)124
Programming language: Fortran.
Computer: IBM RISC System/6000.
Operating system: AIX 3.2, UNICOS 7.0, OSF/1.
RAM: 9999K words
Word size: 64
Keywords: Solid state physics, Crystal field, Green function, Adsorbate, Layer kkr, Dft-lda, Total energy, Hellmann-feynman forces.
Classification: 7.3.

Nature of problem:
The computer code allows to calculate the Green function of an adsorption problem with a single, isolated adsorbate atom (so-called "adsorbate system") on a semi-infinite metal surface. The following physical quantities are available as output: change in electron density for the adsorbate system, change in density of states, total energy of the adsorbate system, and the Hellmann-Feynman forces on the adsorbate atom. The program uses density-functional theory within the local- density approximation for the exchange-correlation functional and ab initio, norm-conserving pseudopotentials.

Solution method:
The Green function of the clean substrate (so-called "reference system") has to be calculated in advance. This reference Green function is needed as input for this code. It is obtainable with the layer KKR method [1,2,3] and has to be projected onto a localized basis of Gaussian orbitals [4,5]. The code described below then solves the Dyson equation self-consistently for the effective potential of the adsorbate atom with the projected Green function of the reference system.

Restrictions:
At this time, only one single adsorbate atom can be handled by the code, although the input is made for a finite number of adsorbate atoms. For the evaluation of the exchange-correlation functional the electron- density change, Delta n**v(r), is evaluated on a mesh in real space. This mesh is restricted to be of cubic shape. The treatment of f- electron systems is not possible with the present code, although there are no limitations in principle.

Running time:
One iteration on a CRAY Y-MP (single processor) takes 82 seconds, on an IBM RS/6000-350 it takes 493 seconds. About 40 iterations are necessary to converge a typical problem which has a linear dimension of 108 in the Gaussian basis and about 40**3 points in the real-space mesh for Delta n**v(r). There are three time consuming parts: * Solution of the Dyson equation. * Projection of the effective potential onto Gaussians. * Transformation of the density matrix (in the Guassian basis) to the real space mesh.

References:
[1] F. Maca and M. Scheffler, Comput. Phys. Commun. 38(1985)403, 47(1987)349.
[2] F. Maca and M. Scheffler, Comput. Phys. Commun. 51(1988)381.
[3] B. Wenzien, J. Bormet, and M. Scheffler, Green function for crystal surfaces I. Submitted to Comput. Phys. Commun.
[4] Ch. Droste, Ph. D. thesis, Fachbereich 4 (Physik) der Technischen Universitat Berlin (1990).
[5] B. Wenzien, J. Bormet, and M. Scheffler, Green function for crystal surfaces II: Projection onto Gaussian orbitals. To be published.