Björn Herschend

Application of the method of increments to the adsorption of CO on the CeO2(110) surface

We have combined an embedded-cluster model with an extension of the method of increments to treat the adsorption of molecules on a surface. In this way we are able to investigate the physisorption of CO on CeO(2)(110) at the MP2, MP4(SDTQ), and CCSD(T) levels with only moderate computational costs. We find that, at the CCSD(T) level, 25% of the adsorption energy originates from electron correlation. The interactions of the CO molecule with its five nearest cerium and oxygen neighbors in the surface layer make the largest contributions to the electron correlation. Approximately 97% of the adsorption-induced electron correlation energy part of the adsorption energy is recovered by the method of increments (in our chosen expansion), at the MP2 level.

The adhesion properties of the Ag/α-Al2O3(0001) interface: an ab initio study

Ab initio computer simulations of the atomic and electronic structure of the Ag/a-Al2O3(0 0 0 1) (corundum) interface have been performed for a periodic two-dimensional slab model using the Hartree–Fock method and a posteriori electron correlation corrections.We have considered both Al- and O-terminated corundum substrate surfaces.The dependence of the adhesion energy on the interfacial distance has been analyzed for the two most favorable Ag adsorption positions over corundum and for two different metal coverages (a 1/3 monolayer (ML) of the Ag(1 1 1) crystallographic plane and a full Ag(1 1 1) monolayer).The two different terminations (Al- and O-) give rise to qualitatively different results.The former case corresponds to the most stable termination of the pure corundum (0 0 0 1) substrate where small adhesion energies per Ag atom (0.15–0.25 eV for 1 ML and 0.40–0.55 eV for 1/3 ML) are accompanied by minor interfacial charge transfer, indicating physisorption, which may be explained by a weak atomic polarization.In contrast, for O-terminated corundum, substantial adhesion energies (3–5 eV per Ag atom at 1 ML coverage and 6–11 eV for 1/3 ML) combined with noticeable charge transfer from silver atoms towards the substrate (0.5e to 0.9e) are clear indications of a strong interfacial ion bonding.For both terminations, the observed difference in Ag adhesion energies for 1/3 ML and 1 ML coverages arises from a transition from directed Ag–O bonding towards a more delocalized electron density distribution in the complete monolayer.The results of our calculations are compared with available experimental studies and theoretical simulations for various Me/Al2O3 interfaces. 2002 Elsevier Science B.V. All rights reserved.

A combined molecular dynamics+quantum mechanics method for investigation of dynamic effects on local surface structures

A combined molecular dynamics (MD)+quantum mechanics (QM) method for studying processes on ionic surfaces is presented. Through the combination of classical MD and ab initio embedded-cluster calculations, this method allows the modeling of surface processes involving both the structural and dynamic features of the substrate, even for large-scale systems. The embedding approach used to link the information from the MD simulation to the cluster calculation is presented, and rigorous tests have been carried out to ensure the feasibility of the method. The electrostatic potential and electron density resulting from our embedded-cluster model have been compared with periodic slab results, and confirm the satisfying quality of our embedding scheme as well as the importance of applying embedding in our combined MD+QM approach. We show that a highly accurate representation of the Madelung potential becomes a prerequisite when the embedded-cluster approach is applied to temperature-distorted surface snapshots from the MD simulation.

Oxygen vacancy formation for transient structures on the CeO2(110) surface at 300 and 750 K.

Ab initio embedded-cluster calculations have been performed for the CeO2(110) surface using temperature induced structures from molecular dynamics (MD) snapshots. As a first step towards understanding how temperature induced distortions of the surface structure influence the surface oxygen reactivity, the energy cost of removing an O atom from the surface was calculated for 41 snapshots from the MD simulation at 300 K. The quantum mechanical embedded-cluster calculations show that already at 300 K the dynamics causes significant fluctuations (root mean square of 0.37 eV) in the O vacancy formation energy (Evac) while the distribution of the two excess electrons associated with the vacancy is virtually unaffected by the surface dynamics and remains localized on the two Ce ions close to the vacancy. It is also found that the quantum mechanical Evac fluctuations can be reproduced by oxygen vacancy calculations using only the relaxed shell-model force field (FF) itself and the MD geometries. Using the FF as the interaction model, the effect of raising the temperature to 750 K and the effect of doping with Ca were investigated for the oxygen vacancy formation.