Effects of Electron Correlations and Structure on Cluster Magnetism

4 Effects of Electron Correlations and Structure on Cluster Magnetism 

For small v compact structures are obtained which have maximal average coordination number z t 0). For example, for v = 2, the ground-state structures are the triangle, tetrahedron, triangular bipyramid, capped bipyramid and pentagonal bipyramid for N = 

These conclusions illustrate the importance of correlations in low-dimensional systems [43-45, 51]. Similar trends are expected to hold for the realistic spd-band model given by Eq. (1). However, it should also be noted that the Hubbard model with the restriction to one orbital per site tends to exaggerate the effects of quantum fluctuations. Extensions of the model by including either several bands or nonlocal interactions should tend to weaken strong fluctuation effects. Moreover, the validity of the Hartree-Fock approximation is considerably improved by using effective interaction parameters that are fitted to experimental bulk results like the bulk magnetic moment /Xb and thus include part of the electron correlations. 

It is a pleasure to thank Prof. J. Dorantes-Davila and Dr P. Jensen for numerous helpful discussions. The authors acknowledge support by the J. S. Guggenheim Memorial Foundation (GMP) and by the Deutsche Forschungsgemeinschaft (KHB). 

Pastor, in Current Problems in Condensed Matter Theory and Experiments, ed. 

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