The nature of the chemical bond in simple oxides: A theoretical journey from the ionic model to the ab initio configuration interaction approach

Abstract

Articles you may be interested inA new model of chemical bonding in ionic melts J. Chem. Phys. 136, 164112 (2012); 10.1063/1.4705668Bound states of positron with urea and acetone molecules using configuration interaction ab initio molecular orbital approach Structures of the aluminum oxides studied by ab initio methods with natural bond orbital analysis Ab initio cluster model wave functions of increasing complexity have been obtained for alkaline-earth oxides MgO to BaO. Using a wave function corresponding to the superposition of the electronic densities of the cations and anions obtained in a Madelung field, an ab initio version of the ionic model is obtained. This simple ionic model is improved with self-consistent field (SCF) and large multireference configuration interaction (CI) wave functions. Analysis of these different types of wave functions shows that the ground state of these oxides is strongly ionic with the ideally ionic configuration having a weight of ::::::95% in the total CI wave function. With all the criteria that we have used, the degree of charge transfer from 0 2 -to M2+ is always very small. Furthermore, the instantaneous electron-electron interactions (correlation effects) treated in the CI wave function have been found to be mainly intra-atomic and especially important for the 2p electrons of 0 2 -. Point charges were used to represent the contribution to the Madelung field made by the atoms not explicitly included in the cluster; they were chosen to reproduce the Madelung field arising when a fully ionic crystal is assumed. Sets of scaled point charges which correspond to a smaller Madelung field were also used. The cluster model results were not significantly changed when the point charges were reduced by as much as a factor of 2 from the values for the fully ionic crystal. This is strong evidence that the ionicity of the crystals results from chemical forces and is not due to the use of an assumed Madelung field external to the cluster.

Good performance of the M06 family of hybrid meta generalized gradient approximation density functionals on a difficult case: CO adsorption on MgO(001)

“…Neyman et al 18 studied the interaction of CO on MgO͑001͒ using two GGA functionals, namely, BLYP 39,40 and BP86, 39,41 and found, in agreement with known trends for other systems, 42 that the simpler LDA density functionals give a systematic overbinding, as reported in their previous works. Although MgO is almost a fully ionic compound, 35,48 calculations using PCs equal to Ϯ1.8 a.u., which quantitatively reproduce the Madelung field outside the MgO͑001͒ surface, yielded smaller frequency shifts similar to the 31 cm −1 shift previously reported by Pacchioni et al 16 However, a further complication is that the reported experimental values of 14 and 35 cm −1 for the vibrational shift correspond, in the former case, to MgO thin films grown on a metallic support and, in the latter case, to MgO polycrystalline samples. Here, a new element entered into the discussion, namely, the vibrational frequency of the adsorbed CO molecule.…”

Character of the electronic ground state and of charge‐transfer excited states in ionic solids: An ab initio cluster model approach

“…For A1203, we find C, = 0.9192 or that I&) accounts for 84.5% of I&-, ). From this analysis, a lower bound for the net charge on atomic A1 will be +2.55, or 85% of the formal 3+ charge, whereas a more reasonable estimate will be +2.7 [36,37]. We must point out that the charge on atomic A1 in bulk corundum is still a matter of controversy and that theoretical estimates are based either on semiempirical calculations or on periodic Hartree-Fock calculations using rather limited basis sets [62].…”

“…In fact, the use of the VB-CI permits us to distinguish inter-and intra-atomic excitations because the orbitals used to construct the configurations are local (see [37]). Moreover, the use of a VB-CI permits a reasonable starting point to describe the wave function associated to charge-transfer excited states, as those described by the resonating valence bond components schematically shown in Eq.…”

“…This is because not all the I$k) determinants included in our CI expansion correspond to charge-transfer excitations. Many of them correspond to intraunit excitations, leading to a very large correlation energy, mainly atomic in origin [37].…”

Electronic band structure of calcium oxide

“…A competing picture is based on a model postulating delocalization of one electron transferred from the cation to the anion (Ca 2+ O -plus e -) [4,5]. The alternative model is supported by two factors: (a) the low-lying empty 3d orbitals of the cation [6] and (b) the instability of the O 2-ion due to an insufficiently strong Madelung potential [7].…”