Compact model potentials for ab initio embedded cluster calculations. Part I. Basic formulation

Abstract

Articles you may be interested inAb initio model potential calculations on the electronic spectrum of Ni2+doped MgO including correlation, spin-orbit and embedding effects Excitedstate absorption spectra of V 2+doped fluoroperovskites. An ab initio model potential embeddedcluster studyIn light of the theory of separability of many-electron systems, a methodology to perform embedded cluster calculations in order to study localized phenomena in a large electronic system is reported. A general equation for the cluster spin orbitals under the assumptions of frozen environment and cluster-environment strong orthogonality requirements is presented. The interactions between active and frozen electrons are adequately introduced through computationally convenient ab initio compact model potentials ͑CMPs͒ built up as a sum of long-and short-range terms. The short-range term, Coulomb and exchange interactions, are developed as a spectral representation in a nonorthogonal basis set. Cluster wave function, energy, and analytic geometrical derivatives calculations with CMPs have been implemented for several kinds of cluster wave functions. In order to evaluate the accuracy of the results obtained using this methodology, some simple test calculations for different hydrogen molecule-cation complexes have been performed both at the Hartree-Fock and multiconfigurational self-consistent-field levels of theory. As shown by an analysis based on the constrained space orbitals variations method, the main deviations are due to the lack of charge transfer contributions in the CMP calculations.

Cluster modeling of metal oxides: case study of MgO and the CO/MgO adsorption system

“…In general, the DGs calculated with the SPC model are in better agreement with the experimental value of 7.8 eV [51]. It is worth noting that the smallest change in the charge distribution upon embedding occurs on (MgO) 9 . This suggests that cluster-lattice interaction is weakest for (MgO) 9 among this series of cluster models, and that the bare-cluster model, if properly chosen, may provide a reasonable description of the solid.…”

“…In bare (MgO) 5 (9, 1), the dangling bonds lead to a too high-lying HOMO, which brings about a too low DG, while in bare (MgO) 13 (21,5), the small DG results from the overstabilized LUMO. Compared with the experimental DG value of 7.8 eV [51], the result of 8.56 eV calculated from cubic (MgO) 9 (9,9) within bare-cluster models seems reasonable.…”

“…In MgO bulk solid, the charges on Mg or O should all be the same and the ratio between the absolute charge on Mg and that on O should be 1:1. For bare-cluster models, this stoichiometry as well as the neutrality requirement is only roughly ful®lled by the cubic (MgO) 9 (9,9) cluster. This is not surprising as we recall that the topologic parameters N and b predict that the cubic model is the best model and those models with ions of coordination numbers less than 2 should be avoided.…”

“…2). These clusters are (MgO) 5 (5, 5), (MgO) 5 (9, 1), (MgO) 9 (9,9), (MgO) 13 (13,13), and (MgO) 13 (21,5). (The numbers of atoms in each layer are given in parentheses.)…”

Improved embedded molecular cluster model

“…Particular cases of such subsystems are, for example, cluster and the remaining part of crystal, fragment of a molecule and the remaining part of it, valence and core electrons, etc. At present, approaches employing the strong orthogonality constraint have been developed [2,4,5,10] and used successfully [3][4][5][6][7]. However, nonorthogonal wave functions of the crystal SE may turn out to be more localized than orthogonal ones.…”