Cluster embedding in an elastic polarizable environment: Density functional study of Pd atoms adsorbed at oxygen vacancies of MgO(001)

Adsorption complexes of palladium atoms on F-s, F-s(+), F-s(2+), and O2- ce nters of MgO(001) surface have been investigated with a gradient-corrected (Becke-Perdew) density functional method applied to embedded cluster models . This study presents the first application of a self-consistent hybrid qua ntum mechanical/molecular mechanical embedding approach where the defect-in duced distortions are treated variationally and the environment is allowed to react on perturbations of a reference configuration describing the regul ar surface. The cluster models are embedded in an elastic polarizable envir onment which is described at the atomistic level using a shell model treatm ent of ionic polarizabilities. The frontier region that separates the quant um mechanical cluster and the classical environment is represented by pseud opotential centers without basis functions. Accounting in this way for the relaxation of the electronic structure of the adsorption complex results in energy corrections of 1.9 and 5.3 eV for electron affinities of the charge d defects F-s(+) and F-s(2+), respectively, as compared to models with a bu lk-terminated geometry. The relaxation increases the stability of the adsor ption complex Pd/F-s by 0.4 eV and decreases the stability of the complex P d/F-s(2+) by 1.0 eV, but it only weakly affects the binding energy of Pd/F- s(+). The calculations provide no indication that the metal species is oxid ized, not even for the most electron deficient complex Pd/F-s(2+). The bind ing energy of the complex Pd/O2- is calculated at -1.4 eV, that of the comp lex Pd/F-s(2+) at -1.3 eV. The complexes Pd/F-s and Pd/F-s(+) exhibit notab ly higher binding energies, -2.5 and -4.0 eV, respectively; in these comple xes, a covalent polar adsorption bond is formed, accompanied by donation of electronic density to the Pd 5s orbital. (C) 2001 American Institute of Ph ysics.