Elastic polarizable environment cluster embedding approach for water adsorption on the α-Al₂O₃(0001) surface. A density functional study

Low coverage water adsorption on the α-Al₂O ₃(0001) surface has been studied with a generalized-gradient density functional approach using embedded cluster and periodic slab methodologies. An advanced cluster embedding method in an elastic polarizable environment (EPE), which enables an accurate description of the adsorption-induced substrate relaxation, has been applied systematically at various density functional levels: PW91, BP, and PBEN. In addition, periodic slab model calculations based on the PW91 functional were carried out for varying surface supercell sizes, (2 × 2) and (3 × 3), which compare very well with the corresponding embedded-cluster results. In agreement with two recent studies employing integrated MO + MO (IMOMO) embedded cluster and periodic Car-Parrinello BLYP methodologies, our calculations predict the 1,2-dissociative adsorption to be about 10 kcal mol-1 more favorable than molecular adsorption; however, at variance with the latter study, we predict 1,4-dissociative adsorption to be least favorable. Analysis of adsorbate-induced relaxation renders the interaction energy with the unrelaxed substrate in the 1,4-dissociative case negative (unbound complex), thus rationalizing the smallest (by absolute value) interaction energy. Our best estimates for binding energies, at the PBEN level, for molecular, 1,2-dissociative, and 1,4-dissociative adsorption are -22.5, -31.2, and -17.2 kcal mol^-1, respectively.