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 been studied using gradient-corrected density-functional calculations. Our technique is based on ultrasoft pseudopotentials, residuum minimization techniques 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 periodically repeated slabs with up to six-layer slabs, allowing for the relaxation of the uppermost layer. For Ni(1 0 0) and Ni(1 1 0) surfaces an adsorption with the centre of the aromatic ring placed above the hollow position has been identified to be energetically most favourable, whereas for the Ni(1 1 1) surface adsorption in the bridge position results in slightly higher binding energies. Adsorption-induced distortions of the molecular geometry are found to be modest in all cases: the C-C bond distances are slightly elongated, but the differences in the bond lengths never exceed 0.03 Angstrom. 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 electronic structure of the adsorbate/substrate complex and of the charge flow induced by the adsorption. Our results an discussed in relation to recent experiments and other theoretical studies. (C) 2001 Elsevier Science B.V. All rights reserved.