Quantum Chemical Studies of Adsorbate-Surface Interactions: Application of Cluster Models to Aluminum on Graphite, Slab Models to Hydrogen on BERYLLIUM0001) and Development of a Novel Embedded Cluster Models

Ab initio quantum chemical studies of various adsorbate-surface interactions are presented. 1. Aluminum adsorption on the basal plane of graphite is studied using the cluster model approach. One layer clusters find the Al prefers to adsorb at the center of the carbon rings. A two layer cluster determines the most stable Al adsorption site to be above the carbon atom in agreement with Scanning Tunneling Microscopy (STM) experiments. 2. Five optimal structures for the Al-Benzene complex have been theoretically determined: (1) Al centered (C_{6v}), (2) Al boat centered (C_{2v} ), (3) Al on-top (C_{s}), (4) Al sigma-bridging (C_{s}), and (5) Al pi-bridging (C_ {s}). Structures (4) and (2) are most likely to be the forms of the complex observed in low temperature electron spin resonance (ESR) experiments. Structure (3) with Al bonding on-top a single C atom is also interesting since it may be prototypical for the chemisorption interaction of an Al atom on graphite. 3. To reduce the computational requirements of the calculations the semiorthogonalization scheme is investigated and it is shown that a systematic method for neglecting two-electron integrals can be developed. 4. An ab initio Hartree-Fock crystal orbital study to model H adsorption on the Be(0001) surface at different coverages is presented. Both single (1L) and triple (3L) layer Be slabs with H on one of the four possible high symmetry sites, on-top, bridge, and three-fold open and eclipsed, are considered. At 1/2th H monolayer coverage on the 1L slab, we find the bridge site to be the favored site. This agrees with electron energy loss spectroscopy (EELS) experiments which assign H atoms to bridge sites when there is low surface coverage. 5. A new embedded cluster procedure which circumvents cluster edge effects is developed. The method couples ab initio cluster calculations with an extended substrate periodic Hartree-Fock calculation. A general symmetry based localization procedure on the extended substrate produces a basis set in the cluster region. The method is illustrated by calculations which simulate H adsorbed at an on-top Li(100) surface site.