Hans-Ottmar Beckmann

Ab initio CI investigation of the interaction of a hydrogen atom with clusters which model the (100) and (110) surfaces of bcc Li-lattice

Abstract Correlation effects are important for the proper description of the electronic structure of the clusters which can serve as models for the interaction between an H atom and the (100) and/or (110) bcc Li surfaces. A majority of the clusters investigated exhibits lowest singlet and lowest triplet states with almost degenerate energies. The models used to describe the interaction of hydrogen with lithium surface show that a dissolution of hydrogen atoms inside the Li lattice in interstationary positions is very probable when the less “compact” (100) bcc surface is exposed to hydrogen. The more “compact” (110) surface should also allow the entry of a H atom inside the Li lattice although under energetically less favorable conditions. These consequences of the study with rigid Li-cluster models would certainly be even more pronounced if the relaxation effects were taken into account. It is generally observed that a smaller number of nearest neighbors in the plane perpendicular to the line of approach of the H atom favors stronger bonds to the hydrogen atom. This fact can cause higher surface activity if irregularities in the form of “peaks”, “ridges” or “hills” are present in the surface. The higher surface activity is indicated by an increase in the H concentration inside the region of the irregularities. This effect is enhanced when the surface irregularities have a high degree of biradicaloid character.

Investigation of alkali-metal clusters with pseudopotential multireference double-excitation configuration interaction method

Abstract Dimers and tetramers of Li, Na and K are studied utilizing an “ab initio” pseudopotential method followed by extensive multireference Cl. Results for Li and Na are compared with corresponding all-electron calculations. Geometry, optimization of K 4 predicts a comparable stability for singlet rhombic and T-shaped forms and for the triplet square arrangement.

Ab initio SCF and CEPA investigations of stable lithium clusters

Abstract Various approaches of two Li 2 molecules are investigated in a search for stable Li 4 cluster configurations and favourable pathways forming them. The lowest energy minimum is found for a singlet state in a rhombic structure with its short diagonal close to the equilibrium internuclear distance of free Li 2 . The stability of this arrangement is traced to the binding potential of the p—π functions of its central Li 2 substructure. This suggests a flat square bipyramidal configuration for Li 6 which is indeed found to represent a local minimum. Binding energies for 2Li 2 → Li 4 and Li 2 + Li 4 → Li 6 are calculated as 15 kcal/mole and 24 kcal/mole, respectively. Formation of both clusters is possible without surpassing energy barriers. Harmonic vibrational frequencies are given for Li 4 and ionization potentials are calculated for Li 4 and Li 6 .

MRD Cl calculations of some properties of the Li3 cluster

Abstract Ab initio SCF and MRD Cl calculations of the Li3 cluster have been performed to determine electronic correlation effects. The calculated binding energy and ionization potential agree with the CEPA values given by Gerber and Schumacher. The eight lowest excitation energies are determined.

Electronic structure and reactivity of the transition-metal lithides ScLi, CuLi and PdLi

Abstract Non-empirical pseudopotential CI calculations were performed on ScLi, CuLi and PdLi diatomics. The reactivity of these transition-metal lithides was tested in the case of interaction with hydrogen. For collinear approaches the bonding with H is weaker than in MH or LiH diatomics (M = Sc, Cu, Pd), particularly when the MLi molecule is stable (CuLi). The HScLi system represents a special case since this molecule is particularly stable. The attack of H in the bridge position seems to be preferred if relaxation of the MLi distance is allowed.

Ab initio studies of He–Ni and He–Cu interaction potentials

Helium diffraction data from the nickel (110) surface has indicated a very small corrugation which decreases with increasing incident energy of He atoms while for copper a larger corrugation effect is observed. None of the theoretical calculations based on surface electronic charge density reported to date have been successful in reproducing the value or the trend for Ni(110). It is suggested that the differences between Ni and Cu are due to the unfilled d shell of Ni. Configuration interaction calculations on He–Ni14 and He–Cu14, using an embedding theory, have been performed for low lying electronic states to determine interaction potentials and softness parameters. For Ni the interaction potentials are found to be similar for atop atom and midbond He trajectories, while for Cu, the atop atom site is much more repulsive in accord with the He diffraction data.

Some comments on the stable forms of small alkali metal clusters

Some general rules on the stable geometrical forms of small alkali metal cluster are formulated and rationalised. The agreement of results of recent quantum chemical investigations, which significantly differ in the methods employed, document the general validity of these rules.

AB initio studies of HeNi and HeCu interaction potentials

Abstract Cluster model configuration interaction calculations on HeNi 14 and HeCu 14 , have been performed for determining helium surface interaction potentials. For Ni, the interaction potentials are found to be similar for atop atom and mid-bond He trajectories, while for Cu, the atop atom site is much more repulsive in accord with the He diffraction data. The calculated He-Cu potentials are fit to within 15% by V = αρ , for a given vertical distance above the surface, where ρ is the substrate charge density. For HeNi, the variation in a is greater.