Theoretical studies of surfaces, clusters, and the many-body problem using semiempirical models

Abstract

Hydrogen chemisorption, the magnetism of small metal clusters, and the many-body problem were studied with the use of the Hubbard Hamiltonian. The chemisorption of hydrogen on transition metals was studied within the unrestricted Hartree-Fock approximation. The results show that the chemisorption energy depends weakly on the initial magnetization of the substrate and adsorbate, in agreement with recent experimental work done by Ertl's group. The magnetism of small metallic clusters was studied using the exact diagonalization procedure. The calculations, done for 5-site clusters, show a new interesting phenomenon involving spin frustration. The results indicate other many-body effects like the resonating valence bond state and singlet and triplet pairing. The many-body calculations were performed using various decoupling procedures, Gutzwiller variational schemes, and the connected-moment expansion. The performance of these techniques was tested and compared to exact results for small Hubbard clusters.