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The LCGTO—Xα Method for Transition Metal Model Clusters

1989; American Chemical Society; Linguagem: Inglês

10.1021/bk-1989-0394.ch013

1947-5918

Notker Rösch, Peter Knappe, P. Sandl, Andreas Görling, Brett I. Dunlap,

Molecular Junctions and Nanostructures

Transition metal clusters as models for surface phenomena are discussed and contrasted with gas phase clusters. The accuracy of ab initio and local density methods for transition metal systems is briefly compared. The linear combination of Gaussian-type orbitale (LCGTO) Xα approach is presented as an efficient method to calculate the electronic structure of such systems within the local density functional theory. Gaussian broadening of the one-electron levels is used to both mimic the embedding of a cluster in a surface and to reduce the number of different SCF calculations which otherwise would be required due to the high density of states around the highest occupied (Fermi) level. The method is applied to nickel clusters of up to 17 metal atoms. In a comparative study of compact clusters it is found that an icosahedral structure is energetically favored for a Ni13 cluster over a cuboctahedron, a small piece of crystalline nickel. Spin-polarization is calculated to be 0.62 μB per metal atom for both structures investigated, a value that is rather close to bulk magnetization. Na and K chemisorbed on rather planar Nin clusters model the low coverage limit of alkali chemisorption at various sites of different crystalline surfaces. Calculated values for binding energy and induced 'surface' dipole moment show satisfactory agreement with experimental data. From the calculated adsorbate-substrate bond length a substantial covalent character of alkali bonding to transition metal surfaces is deduced in the low coverage limit. A spin-polarized calculation shows that the unpaired spin of the alkali atom is almost completely quenched upon chemisorption.