Density-functional study of the adsorption of benzene on the (111), (100) and (110) surfaces of nickel

The adsorption of benzene on all three low-index surfaces of nickel has bee n studied using gradient-corrected density-functional calculations. Our tec hnique is based on ultrasoft pseudopotentials, residuum minimization techni ques for the calculation of the electronic ground-state and of the Hellmann -Feynman forces and stresses, and on a conjugate-gradient technique for the optimization of the atomic structure. The surfaces have been modelled by p eriodically repeated slabs with up to six-layer slabs, allowing for the rel axation of the uppermost layer. For Ni(1 0 0) and Ni(1 1 0) surfaces an ads orption with the centre of the aromatic ring placed above the hollow positi on has been identified to be energetically most favourable, whereas for the Ni(1 1 1) surface adsorption in the bridge position results in slightly hi gher binding energies. Adsorption-induced distortions of the molecular geom etry are found to be modest in all cases: the C-C bond distances are slight ly elongated, but the differences in the bond lengths never exceed 0.03 Ang strom. The aromatic ring remains hat, but the H atoms are tilted away from the surface of the substrate. We also present a detailed analysis of electr onic structure of the adsorbate/substrate complex and of the charge flow in duced by the adsorption. Our results an discussed in relation to recent exp eriments and other theoretical studies. (C) 2001 Elsevier Science B.V. All rights reserved.