CO/MgO(001) at different CO coverages: a periodic ab initio Hartree-Fock and B3-LYP study

The adsorption of CO at various coverages on the regular MgO(0 0 1) surface has been studied using a rigorous ab initio approach based on the solution of the periodic Schrodinger equation at both Hartree-Fock and DFT levels. The crystal orbitals are expanded in Gaussian atomic orbitals and the Coulomb and exchange series are treated rigorously as implemented in the CRYSTAL-98 computer code, ensuring the correct behavior of electric field outside the crystalline surfaces. For DFT calculations, the B3-LYP functional coupled with Gaussian basis sets of high quality for periodic calculations have been adopted. Three different CO coverages (namely 1 x 2(CO x Mg), 1 x 4 and 1 x 8) have been considered. Because of the very small binding energies (BEs), a careful analysis of the effects of the basis set superposition error as well as the computation of the harmonic CO frequency shift with numerical procedures of better quality than in the past have been carried out. Comparison with the results obtained for the CO/Na+ and CO/CO2 complexes has also been made in order to clarify the bonding at ionic surfaces. Neither charge transfer nor polarization play a significant role, and the weak BE results from a large cancellation between electrostatic and exchange repulsion components as formerly suggested by accurate cluster calculations embedded in total-ion ab initio model potential. Our best binding energy and CO frequency shift are still underestimated with respect to the most updated experimental data and in good agreement with the best data from embedded cluster calculations. The disagreement with experiment seems to suggest that the dispersive contribution to the BE, unfortunately not accounted for by the Hamiltonians encoded in the available computer periodic codes, may play an important role to reconcile the data of computer simulation with those From experiment. (C) 2001 Elsevier Science B.V. All rights reserved.