Comparison between local space approximation and finite cluster treatments of chemisorption on metals

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Abstract
The local space approximation (LSA) method is applied to a model Hamiltonian describing the chemisorption of a hydrogen atom onto a transition metal (0,0,1) surface. In the LSA method a finite cluster defined by a local (orbital) space is properly embedded into the complete system. Without embedding the calculated binding energy, ΔEb oscillates strongly as the size of the cluster increases, and even when 200–250 metal atoms are included the convergence is unsatisfactory. On the other hand, the LSA treatment yields rapid monotonic convergence. Using a systematic buildup of the local space the error in ΔEb is 4.4 kcal/mole out of 80.5 for a 10 metal atom cluster; that error decreases steadily to 1.8 kcal/mole for a cluster containing 30 metal atoms. Extrapolation reduces the error for all clusters between 18 and 71 metal atoms by about 0.6 kcal/mole.