Density functional cluster description of ionic materials: Improved boundary conditions for MgO clusters with the help of cation model potentials

Abstract

ABSTRACT:Ionic materials can be conveniently described by quantum mechanical cluster models. In these models the electrostatic field of the crystal surrounding is often Ž . taken into account by embedding the cluster in an array of point charges PCs . However, anions at the cluster boundary can be strongly polarized by neighboring positive PCs. These artifacts can be significantly reduced if a model potential description is employed for the nearest-neighbor cations of the surrounding. We have implemented a scheme of Ž . total ion model potentials TIMP in the density functional code LCGTO-DF. In the Ž . present work we compare a number of electronic properties of MgO 001 models, like core-level splittings, electron affinity, and electrostatic potential, computed with the help of TIMP embedding to results obtained for the same clusters surrounded by PCs only. The cluster models range from MgO 8y to Mg O . Adsorption of a CO molecule on 5 2 5 2 5TIMP embedded cluster models of a regular surface site and the three-coordinated corner Mg 2q site is also considered.

The role of artificial polarization in FA2:Ca2+ lasers and NO interactions at RbCl (001) surface: An ab initio study

“…Surface reaction of the heterogeneous catalysis often proceeds via the following series of elementary steps: (1) adsorption of the reactants on the surface; (2) conversion between adsorbates; and (3) desorption of the product from the surface. Study on the adsorption [ step (1) above] is essential for understanding of the first step of surface reaction.…”

“…Previous studies [1,2] have demonstrated that the use of point charges for embedding clusters in ionic crystals can significantly affect the calculated adsorption energies when the positive point charges are nearest neighbors to the highly polarizable anions of the clusters. In that case, an artificial polarization of the anions at the cluster borders results and this artifact can be essentially eliminated by surrounding the anions with ''real'' cations instead of positive point charges.…”

Pd diffusion on MgO(100): The role of defects and small cluster mobility

“…Cluster and supercell models using the BP and PW91 density functionals give a binding energy of ca. 1.4 eV [44][45][46]36].…”

“…Recent in situ Grazing Incident X-ray Scattering (GIXS) studies also confirmed that Pd adatoms adsorb at oxygen anions [25,26], in contrast to an earlier surface (extended) electron energy loss fine structure (SEELFS) study which suggested that the Pd adatoms bind to magnesium sites [57]. The Pd-surface distance for monomers is calculated here to be 2.09 Å , consistent with previous calculations [43][44][45][46]. Experimental measurements of 2.22 ± 0.03 Å for 1 ML Pd deposited on MgO(1 0 0) [25,26] are somewhat larger, perhaps due to the weaker Pd-O bonds expected in a Pd monolayer.…”

Density Functional Embedded Cluster Study of Cu4, Ag4 and Au4 Species Interacting with Oxygen Vacancies on the MgO(001) Surface

“…Here, Mg pp* labels pseudopotential centers Mg 2 + , entirely without electrons, which saturate the coordination spheres of O anions at the cluster boundaries. [23] Our models feature C s symmetry, which was exploited in the calculations. [24] Geometry optimizations of selected systems Cu 4 /O 2À , Cu 4 /F s and Cu 4 /F s + completely without symmetry constraints revealed essentially unchanged structures and adsorption energies; normal mode analysis of C 1 models confirmed that the most stable complexes (structures A, see below) indeed correspond to local minima of the potential energy surfaces.…”