Local Isoelectronic Reactivity of Solid Surfaces.

The quantity w (r) = (1/kTel)[∂n(r, Tel)/∂Tel]v(r),N is introduced as a convenient measure of the local isoelectronic reactivity of surfaces. It characterizes the local polarizability of the surface and it can be calculated easily. The quantity w (r) supplements the charge transfer reactivity measured e.g. by the local softness to which it is closely related. We demonstrate the applicability and virtues of the function w (r) for the example of hydrogen dissociation and adsorption on Pd (100). 73.20At, 82.65.Jv, 7120.Cf Typeset using REVTEX 1 Density-functional calculations of chemisorption processes and of potential energy surfaces of the dissociation of simple molecules over surfaces, which have become available in the last years, have greatly improved the fundamental understanding of reactions at solid surfaces. However, those extensive computations are limited to a restricted number of model systems. It remains an important task to develop a methodology that allows the prediction and interpretation of reactions at surfaces in terms of the properties of the non-interacting systems. Such concepts, well known in molecular chemistry, are the essence of “reactivity theory”. They are based on low-order perturbation theory and aim at a description of the early stages of chemical interactions. The ensuing response functions are called “reactivity indices” and characterize the changes of the electronic structure of one reactant as stimulated by the presence of the other or vice versa. In an early contribution to this field Fukui et al. [1,2] established the correlation between the frontier-orbital density, i.e., the density of the highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO), and the reactivity of a system towards electron donation or accptance. Pearson [3] introduced the electronic “softness”, the magnitude of the change of the electronic structure due to a change of the number of electrons in the system, as a measure of the reactivity. Species are classified as “soft” if only a small energy is required to change their electronic configuration, i.e., if the valence electrons are easily distorted, polarized, removed or added. A “hard” species has the opposite properties holding its valence electrons more tightly [3,4]. The utility of the hardness–softness concept is based on the so called hard and soft acid and base (HSAB) principle formulated by Pearson [3] which states that hard-hard (soft-soft) interactions are preferred. In the case of polyatomic or extended systems the HSAB principle is used in a local version: The soft (hard) parts of one reactant prefer to interact with soft (hard) areas of the other [5]. Parr and collaborators [5] gave a foundation in density functional theory to those mostly semi-empirical concepts, and Cohen et al. [6–8] have reviewed the foundations of reactivity theory and addressed some unresolved issues. The (local) softness and hardness describe the response of the electron density to a change of the charge state of the system. For extended, gapless systems the local softness 2