Two LinH (n = 7, 9) clusters have served as models for interaction of a H atom with a stepped Li surface. Potential energy contour maps (or curves) of adsorption and surface diffusion for the LInH system were constructed by using ab initio Hartree-Fock calcns. with a small basis set. The studies of the smaller Li7H cluster showed that there are 3 different "bridge" sites in a direction perpendicular to the ledge in the vicinity of the step. The most stable of these is near the ledge on the upper terrace. The existence of the step distinctly changes the activation energy of surface diffusion. Due to the presence of a higher energy barrier at the ledge, a migrating atom will be reflected in the ledge or trapped on and near the step surface. The surface diffusion path with min. energy was determine Calculation of potential energy curves for the larger Li9H cluster obtained by adding 2 Li atoms in the second layer under the bridge site on the upper terrace revealed that a small decrease of activation energy for surface diffusion occurs. It is rather favorable that a H atom is bound to bridge site on the upper terrace. Relative stabilities at adsorption sites and geometry of the potential barrier are scarcely changed. These studies showed that the presence of the stepped plane induced a perturbation to chemisorption and migration of a H atom. The ledge plays an important role in the migration of adatoms.

Key words: COMPUTATIONAL METHOD, HYDROGEN, LITHIUM COMPOUNDS, CLUSTER COMPOUND, AB INITIO CALCULATION, DIFFUSION, POTENTIAL BARRIER, SURFACE DEFECTS, SURFACES ADSORPTION