MOLECULAR-ORBITAL THEORY FOR CHEMISORPTION - THE CASE OF H ON NORMAL METALS

An ab initio linear combination of atomic orbitals (LCAO) method is presented to calculate the electronic properties of solids. This method is based on the following points: (i) The total solution of the electronic system is obtained using an expansion of various physical parameters up to second order in the overlap between the different atomic orbitals; extensions to large-overlap cases are also discussed. (ii) The total many-body Hamiltonian is reduced to a superposition of Hamiltonian bonds, defined for each pair of atomic orbitals. (iii) The parameters for hopping between two orbitals are related to the Bardeen tunneling currents between the same wave functions; these tunneling currents play, in our approach, the same role as pseudopotentials in the free-electron theory of solids. (iv) Many-body effects are treated using a Slater-like approximation for the exchange and correlation interaction. We show that a many-body Slater-like potential can be introduced within our LCAO approach. Our method has been demonstrated by considering the simple molecules H-2 and LiH. A further application has been made for the chemisorption problem of a hydrogen monolayer adsorbed on the Li(100) and Al(100) surfaces. Results are presented for the chemisorption energies, equilibrium distance of the adsorbed layer, and the density of states. Good agreement is found with other theoretical results and experiment. Our results indicate that the main mechanism for the hydrogen adsorption on simple metals is associated with the lowering of the hydrogen affinity level due to the electrostatic interaction with the metal atoms.