Study of CO adsorption on the Fe(100) surface using the Laplacian of the electronic charge density

The interaction of a CO molecule with the Fe(100) surface is analyzed by the topology of the Laplacian of the electronic charge density −∇ 2 ρ. This analysis shows that the atomic graph of the top atoms of the Fe(100) surface is a cube that exposes a face, F top, with a “hole” of charge or (3,+1) critical point of −∇ 2 ρ, whereas the corresponding graph for the second layer atoms is an octahedron that exhibits a vertex, V sec, protruding above the Fe(100) surface, with a “peak” of charge or (3,−3) critical point of −∇ 2 ρ. Atomic graphs determined for the CO molecule show that the carbon atom has a non-bonded vertex, C Vnb, and a torus, C torus, of charge depletion perpendicular to the C–O bond direction. As the CO molecule approaches the surface, electron transfer towards the tilted CO in turn induces two non-bonded critical points, O Vnb, on the O atom. In accordance with experimental and theoretical results, the topological theory of the Laplacian suggests that the preferred pathway for CO dissociation corresponds to the tilted orientation on the surface, in which the attractive interaction C torus– V sec is enhanced by the interaction of the two O Vnb with the F top on two Fe top atoms of the (100) surface.