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Abstract

The structure of small Rh clusters compatible in size with the particles that are present in highly dispersed supported catalysts are studied at the SCF level by means of intermediate neglect of differential orbital (INDO) calculations. In agreement with experimental determinations and calculations on other metals, we have found nonperiodic (precrystalline) structures of C5v and D5d symmetry the most stable ones. An analysis of these geometries suggest that they result from first- and second-order Jahn–Teller distortions of icosahedra. Although higher multiplicities characterize higher symmetry structures, we find that the lower symmetry structures are characterized by low multiplicities. The electronic characteristics of the surface sites are analyzed at the CI level. In contrast to extended periodic surfaces, which are characterized by negative charge density on the metal atoms, the surfaces of the small clusters appear as electron deficient structures, with negative charge localized on the internal atoms. This may be a key factor in understanding the different catalytic activity of metal surfaces and supported metal catalysts. © 1993 John Wiley & Sons, Inc.