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We use a total energy difference approach to explore the ability of various density functional theory based methods in accounting for the differential effect of static electron correlation on the C(1s) and O(1s) core level binding energies (BEs) of the CO molecule. In particular, we focus on the magnitude of the errors of the computed C(1s) and O(1s) BEs and on their relative difference as compared to experiment and to previous results from explicitly correlated wave functions. Results show that the different exchange-correlation functionals studied here behave rather erratically and a considerable number of them lead to large errors in the BEs and/or the BE shifts. Nevertheless, the TPSS functional, its TPSSm and RevTPSS derivations, and its corresponding hybrid counterpart, TPSSh, perform better than average and provide BEs and BE shifts in good agreement with experiment.

The light-induced spin crossover in FeII complexes is studied with complete active space second-order perturbation theory. The potential energy surfaces of the low-spin and high-spin states are determined as function of the Fe--ligand distance interpolating between the high-spin and low-spin equilibrium geometries. In addition, we calculate the relative energies of the lowest excited states along this coordinate. To address the mechanism of spin crossover, the metal-to-ligand charge transfer states with singlet, triplet, and quintet spin coupling were also treated. The curves of the singlet and triplet MLCT states nearly coincide, but the quintet MLCT curve is higher in energy and the equilibrium distance displaced to larger distances. The possible deactivation mechanisms of the 1MLCT state are discussed considering the relative energies and spin-orbit coupling matrix elements of the relevant electronic states. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem 111:3385–3393, 2011

The optical transition energies of a point defect in MgO(100), consisting of an oxygen vacancy with one trapped electron and an OH − group adsorbed nearby the surface oxygen vacancy (the F s + (OH − ) center), have been studied by means of ab initio multiconfiguration second order perturbation theory (CASPT2) and Density Functional (DFT) calculations on embedded cluster models. We find that a method that takes explicitly into account electronic correlation effects is required to properly predict the first transition energies. The lowest transition energy of the F s + (OH − ) center on a terrace site is very similar to the one reported in previous studies of F + centers on a regular MgO(100) surface (F s + center). Experiments have difficulty distinguishing between these two kinds of point defects based solely on the optical spectra. We have also considered the situation where the vacancy is located at low-coordinated sites like steps and corners. The results show that the loss of coordination decreases the transition energies in agreement with what has been reported in previous studies for “classical” F s + centers.

The effect of spin-orbit coupling is assessed through the spin-orbit state interaction method. This method involves a two-step procedure to include both dynamical electron correlation effects and spin-orbit effects on the relative energies of the low-lying electronic states. Accurate energies of the spin free states are obtained with the complete active space self-consistent field/complete active space second-order perturbation theory (CASSCF/CASPT2) procedure. Second, the spin-orbit coupling is taken into account by interaction of the CASSCF states using the CASPT2 relative energies. The method is validated on the Co2+ ion in vacuum and in the ionic insulator CoO. Afterward the method is applied to CoCl2(PPh3)2 and the dinuclear complexes [TM2Cl6]2− (TM = Mn, Fe, Co). For the monomeric species, the results compare favorable with experimental data or fully relativistic benchmarks. We discuss the difficulties to interpret the dimer results in terms of the parameters that are usually derived from experiment. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

We recently developed a scheme for first-principles calculations of hopping matrix elements between localized states in extended systems. We apply the scheme to the determination of double exchange (DE) parameters in lightly hole-doped LaMnO(3) and electron-doped CaMnO(3). DE is one of the important factors for understanding the properties of doped manganites. The calculations are based on the construction of wave functions for localized hole states or localized electron states for large embedded clusters. The wave functions of these clusters are expressed in terms of localized orbitals, obtained from calculations on smaller units, or "fragments," centered around a transition metal ion. The starting point of electronic states expressed in terms of localized orbital sets is conceptually attractive. It also allows for a rigorous treatment of local electron correlation and electronic relaxation effects. In the present study, the fragments are embedded [MnO(6)] units. The large clusters contain either two or four Mn ions and all neighboring oxygen ligands. The results are compared with conventional embedded cluster calculations. In both compounds, the effective hopping matrix elements, or "double exchange" (DE) parameters, in the ab planes (in the Pbnm space group) are larger than along the c axes. We found nearly perfect agreement with the Anderson-Hasegawa model for the spin dependence of the DE parameters. Nearest-neighbor parameters are more than one order of magnitude larger than next nearest-neighbor parameters. In LaMnO(3) the DE in the ab planes is approximate to -0.26 eV. If there were no Jahn-Teller distortion present in the material, it would have been twice as large. In CaMnO(3), the corresponding nearest-neighbor DE parameter for hopping of a doped electron in the ab planes is only approximate to -0.17 eV, due to the antiparallel spin coupling. However, since this interaction is much larger than the exchange coupling, we suggest that it induces local ferromagnetic clusters around the doped electrons. (C) 2006 Wiley Periodicals, Inc.

The interaction of Cl on different metal surfaces has been studied by density functional theory. The surface is represented by periodic slab models. It is shown that the change on the surface work function induced by the presence of the adsorbed halide can be either positive or negative, depending on the surface metal. While this has already been pointed out for some particular cases such as Cl on Cu(0 0 1) or N on W(1 0 0), we show that this effect may also appear in the case of having the same adsorbate on different metal surfaces. Therefore, one may conclude that the use of the work function change to extract information about the net charge on the adsorbate should be regarded with extreme caution.

A model based on the use of ab initio quantum chemical explicitly correlated wave functions is proposed to study the electronic transfer reaction between an adsorbate and a metallic electrode. In this model, the effects of the electrode potential are simulated through the application of a uniform external electric field. This field exerts an influence on the relative stability of the two electronic states that are mainly involved in the electron transfer. These electronic states have the transferred electron localized either in the electrode or in the adsorbate. The model has been applied to the electron transfer between halides and a cluster model representation of the Cu(100) single-crystal electrode. A linear correlation between the electric field intensity at which the two electronic states involved in the charge-transfer process become degenerate and the normal reduction electrode potential is found. The existence of this linear correlation indicates that the proposed model contains the essential physics governing the trend of halide oxidation at a metal electrode.

Accurate electronic structure calculations of the lowest excited states have been performed on twenty snapshots of a molecular dynamics simulation of [Fe(bpy)3](2+) dissolved in water. The thermal motion distorts the structure of the complex from its average D3 symmetry, causing the localization on one bipyridine ligand of the excited electron in the metal-to-ligand charge transfer (MLCT) state. The excitation energy is about 0.25 eV lower than that for the delocalized description of the MLCT state and is in good agreement with experiments. The composition of the MLCT band is carefully analyzed and the effect of thermal motion on the mechanism of light-induced spin crossover is discussed.

The variations experienced by the energy Eu(π) of the eu(π)b1g (∼x2–y2) charge-transfer transition of (C2H5NH3)2CdCl4:Cu2+ upon pressure in the 0- to 40-kbar range have been measured at room temperature by means of a sapphire anvil cell. These data reveal that Eu(π) undergoes a red shift of 1400 cm−1 on passing from ambient pressure to 40 kbars. To understand this puzzling result theoretical calculations of ∂Eu(π)/∂Req and ∂Eu(π)/∂Rax have been performed where Req and Rax mean the equatorial and axial Cu2+–Cl− distances of the elongated CuCl64− complex, respectively. All results indicate that ∂Eu(π)/∂Req and ∂Eu(π)/∂Rax for Req=228 pm and Rax=297 pm are indeed negative. Moreover ab initio complete active space self-consistent field (CASSCF/CASPT2) and density functional calculations lead to ∂Eu(π)/∂Rax values, which are about 10 times smaller than those of ∂Eu(π)/∂Req. From the ensemble of experimental and theoretical results, it is concluded that a pressure of 40 kbars gives rise to a decrement of ≈25 pm of the axial distance and at the same time to an increase of ≈7 pm of the equatorial one. It is stressed that the present study on a diluted Jahn–Teller impurity lies far beyond the current possibilities of X-ray absorption structure techniques. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

The low-lying electronic transitions in a peroxy radical, wSi-O-O¿, a fundamental radiation induced point defect in silicon dioxide, have been investigated by means of ab initio multiconfiguration perturbation theory calculations, CASPT2, and cluster models. The accuracy of the computed transition energies and intensities as predicted by the CASPT2 approach has been checked by studying the exited state properties of two molecular analogues, the HOO¿ and CH3OO¿ radicals, and of a well-characterized paramagnetic defect in silica, the nonbridging oxygen center, wSi-O¿, for which unambiguous assignments exist. We found that the peroxy radical gives rise to two optical absorption bands, a very weak one at 0.7 eV and an intense one at 5.49 eV, in agreement with the experimental assignments of Radzig [V. A. Radzig, Chem. Phys. Reports 14, 1206 (1995)].

due to the presence of the Ag 0 impurity. Moreover, from the present results, it is predicted that on passing to NaCl:Ag 0 the CT onset would be out of the optical range while the 5s-5p transition would undergo a redshift of 0.3 eV. These conclusions, which underline the different character of involved orbitals, are consistent with experimental findings. The existence of a CT transition in the optical range for an atom inside an ionic host is explained by a simple model, which also accounts for the differences with the more common 3d systems. The present study sheds also some light on the R dependence of the s 2 - sp transitions due to s 2 ions like Tl 1 .

The equilibrium geometry of Ag 0 centers formed at cation sites in KCl has been investigated by means of total-energy calculations carried out on clusters of different sizes. Two distinct methods have been employed: First, an ab initio wave-function based method on embedded clusters and second, density-functional theory ~DFT! methods on clusters in vacuo involving up to 117 atoms. In the ab initio calculations the obtained equilibrium Ag 0 -Cl 2 distance Re is 3.70 A, implying a large outward relaxation of 18%, along with 7% relaxation for the distance between Ag 0 and the first K 1 ions in ^100& directions. A very similar result is reached through DFT with a 39-atom cluster. Both approaches lead to a rather shallow minimum of the total-energy surface, the associated force constant of the A1g mode is several times smaller than that found for other impurities in halides. These conclusions are shown to be compatible with available experimental results. The shallow minimum is not clearly seen in DFT calculations with larger clusters. The unpaired electron density on silver and Cl ligands has been calculated as function of the metal-ligand distance and has been compared with values derived from electron-paramagnetic resonance data. The DFT calculations for all cluster sizes indicate that the experimental hyperfine and superhyperfine constants are compatible whenRe is close to 3.70 A. The important relation between the electronic stability of a neutral atom inside an ionic lattice and the local relaxation is established through a simple electrostatic model. As most remarkable features it is shown that ~i! the cationic Ag 0 center is not likely to be formed inside AgCl, ~ii! in the Ag 0 center encountered in SrCl2, the silver atom is probably located at an anion site, and ~iii! the properties of a center-like KCl:Ag 0 would experience significant changes under hydrostatic pressures of the order of 6 GPa.

Ab initio cluster model wave functions are used to predict the existence of localized excited states in MgO, ${\mathrm{Al}}_{2}{\mathrm{O}}_{3},$ and ${\mathrm{SiO}}_{2}$ arising from metal $2p$ core-level excitations. Theoretical values obtained at different levels of theory result in a quantitative agreement with experiment, and the use of different models permits us to quantify the different contributions to the final excitation energy. The most important contribution is atomic in nature; a meaningful zero-order approximation is that in MgO and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ the exciton can be assigned to a ${M(2p}^{6})\ensuremath{\rightarrow}{M(2p}^{5}{3s}^{1})$-like excitation, where $M=\mathrm{Mg}$ or Al. For the atomic models, the singlet-triplet exchange in the excited configuration is in good agreement with experiment. In addition, the solid-state effects on this exchange energy predicted by experiment are well reproduced by the cluster models representing MgO and ${\mathrm{SiO}}_{2},$ whereas a less clear situation appears in ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}.$ The open-shell orbital in the final state has, however, important contributions from the ions near the atomic site where excitation occurs. Nevertheless, the final state appears to be localized in space without any a priori assumption, the localization following from the hole-particle interaction implicitly induced in the final-state wave function. The Madelung field reduces the excitation energy with respect to the atomic value; the effect of neighboring atoms, mainly Pauli repulsion, acts in the opposite way; and electronic correlation effects decrease it again. In agreement with the covalent nature of ${\mathrm{SiO}}_{2},$ the exciton cannot be simply understood as arising from a $\mathrm{Si}{(2p}^{6})\ensuremath{\rightarrow}\mathrm{Si}{(2p}^{5}{3s}^{1})$ in a fully oxidized Si cation.

We report ab initio calculations on the optical transitions of oxygen vacancies (F centers) located at low-coordinated sites of the MgO (100) surface. F and F + centers at terrace, step and corner sites were investigated by means of cluster models embedded in point charges. The transition energies to the lowest excited states were determined by performing extensive configuration interaction calculations where correlation effects are explicitly taken into account. The transitions due to F centers at the surface of MgO are considerably red-shifted, by ∼2.5 eV, compared to the bulk excitations. We also found a decrease of ∼0.5–1.2 eV of the transition energies of oxygen vacancies at step and corner sites compared to those at the (100) terrace sites. This trend is explained in terms of reduced Pauli repulsion in the excited state. According to these results, F centers at terrace, step and corner sites should give rise to two distinct optical bands. We tentatively suggest that the features at 2.05 and 2.3 eV observed experimentally are due to the low-coordinated step and corner F centers and not to centers at the MgO (100) terraces.

A series of embedded cluster calculations have been performed to study the dependence of the calculated ionization and excitation energies in CuCl and NiO on the representation of the direct cluster surrounding. Different embedding schemes have been applied. First, a cluster of one TM ion plus its nearest counterions has been embedded in point charges only. Next, model potentials (AIEMPs) have been used to represent the first layer of ions around the basic cluster, and finally, we embedded the cluster in the charge distribution of frozen ions. The calculations show that the ionization and excitation energies change considerably when the Pauli repulsion of the cluster atoms with the direct surrounding is accounted for. (C) 1999 Elsevier Science B.V. All rights reserved.

The spectral transitions and the character of the low-lying excited states of the copper halides, CuX (X = Cl, Br, I) are studied by means of two different relativistic computational approaches. One is based on the CASSCF/CASPT2 approach with operators accounting for scalar relativistic effects evaluated as a first order correction to the CASSCF energy. The other is a fully relativistic four component SCF-CI treatment based on the Dirac-Coulomb Hamiltonian and hence accounts intrinsically for spin-orbit coupling as well as for scalar effects. The lowest excited states ((1,3)Sigma(+), (1,3)Pi, (1,3)Delta) are all closely related to the formal ionic configuration Cu+(3d(9)4s(1)) X-(ns(2)np(6)). The agreement between calculated and measured transition energies and transition dipoles and their trends in the series strengthens recent assignments of the observed bands. Unobserved 'neutral' states, dominated by the configuration Cu(3d(10)4s(1)) X(ns(2)np(5)), are situated mostly far above the 'ionic' states. Particular attention was given to the mixing of these states, i.e. to the importance of charge transfer effects in the description of the observed states. These seem to be of significance only for the (1) Sigma(+) states, judging from the weights of the charge transfer configurations in the total wave functions and the character of the open shell orbitals. The calculated increase in charge transfer on going from Cl to I in the series goes together with an increase in the calculated transition dipoles for the (1) Sigma(+) states. This is consistent with the observed decrease of the lifetimes. The magnitudes of the spin-orbit splittings in the ionic states are governed by the splitting in Cu+ (2000 cm(-1)) as expected.

The character of the low-lying excited states of diatomic CuCl is studied primarily by means of the complete active space self-consistent field (CASSCF), method and a second order perturbation approach with the CASSCF wave function as reference state [complete active space perturbation theory to second order (CASPT2)]. Far comparison, the lower levels of the spectra of the Cu+ ion are also analyzed. A first order treatment of the scalar relativistic effects, the mass-velocity and Darwin terms, is included in the calculations. The importance of spin-orbit interactions is investigated by comparing our nonrelativistic valence shell CI (VCI) and relativistic results obtained with our four-component program suite MOLFDIR. The six lowest excited states of the CuCl molecule, which are related to the Cu+(3d(9)4s(1))Cl-(3s(2)3p(6)) ionic configuration, are assigned. The assignments agree with earlier theoretical work. Where they can be compared, the calculated spectroscopic constants are in good agreement with the experimental data. (C) 1997 American Institute of Physics.

Ab initio cluster model wavefunctions for bulk, surface and defect sites of crystalline MgO have been obtained in order to determine possible variations in the degree of ionicity in different physical environments. The degree of ionicity has been estimated from two different and complementary theoretical techniques. On the one hand, the constrained space orbital variation method allows us to quantify the importance of the different energetic contributions to the total energy. Also, projection techniques applied to the final Hartree-Fock wavefunction, permit us to determine the importance of covalent effects. Our analyses show that MgO behaves as an almost perfect ionic crystal and that there is no significant increase in the covalent contributions on the regular surface sites. On the other hand, less coordinated sites like steps and corners, exhibit a moderate increase of covalency. This behaviour is rationalized in terms of a reduced Madelung field at the defect sites.

In this work we discuss the interaction of an adsorbate on an ionic surface taking as representative example CO on a perfect MgO(100) surface. The main goal is to investigate the different contributions to the interaction and how to model them by using a finite cluster model. To this end we use three different ab initio Hartree-Fock approaches. First, we discuss the convergence properties of the array of point charges used to simulate the Madelung potential. Next, we use large cluster models to show that there is an oscillatory behavior of the interaction energy. We show that rather large clusters are needed to avoid such oscillations. Also, we discuss how to represent these large clusters by using model potentials and compare the results with those obtained from periodic Hartree-Fock calculations. The interaction of CO on MgO is found to be weak and of electrostatic origin, with no noticeable chemical contributions.

The electronic structure of the ground electronic state and of some special charge-transfer excited states in ionic solids is examined from the ab initio cluster model approach. Different ab initio wave functions, including a frozen orbital approach, the Hartree–Fock self-consistent field, and multireference configuration interaction wave functions, are considered and analyzed using different theoretical techniques. We explicitly consider some alkaline–earth oxides such as CaO, a more difficult case such as A12O3, a transition-metal oxide such as NiO, and a system with a more complicated structure such as KNiF3. Analysis of ab initio wave functions in terms of valence bond components shows that all these compounds are largely ionic, thus supporting the simple picture arising from the ionic model. However, the nature of the excited states is more complex. Alkaline–earth oxides lowest excited states are essentially described as charge-transfer excitations dominated by a single resonant valence bond structure and the calculated energy difference is comparable to the experimental optical gap. In the case of A12O3, the electronic spectra presents excitonic features and the local charge-transfer excitation excited states provide a reasonable representation of these phenomena. Finally, several different valence bond structures are present in the lowest electronic states of KNiF3. © 1994 John Wiley & Sons, Inc.

We explore the excited state energy landscape of small polyhedral zinc sulfide clusters (Zn(4)S(4) and Zn(6)S(6)) using time-dependent density functional theory and correlated wave function based methods. We predict the optical absorption and photoluminescence spectra of the polyhedral clusters and demonstrate that, upon relaxation of the excited state, these nanostructures break symmetry and an electron and a hole localize on a small number of Zn (electron) and S (hole) centers. We further test several exchange-correlation potentials for their ability to recover the correlated wave function description of the excited state. Finally, we discuss how the degeneracy of excited states in nanostructures, such as those considered here, results in a Jahn-Teller distortion of the excited state geometry, and how numerical problems arising from this can be circumvented by starting the optimization of excited states from structures distorted along the ground state vibrational normal modes.

Cluster model wavefunctions for the alkaline-earth oxides are analyzed to determine the origin of the trend of the chemical shifts of the binding energies, BEs, of the metal core levels. Two cancelling initial states mechanisms in the oxides are responsible for the trend. One is the increase of the BEs for the dication over the neutral atom. The second is the decrease of the free dication BEs due to the Madelung potential of the ionic oxides. The trend given by the sum of these two contributions to the chemical shifts is in full agreement with experiment. These mechanisms are electrostatic and do not involve chemical bonding effects.

The performance of Atomic Natural Orbital (ANO) basis sets for calculations involving nonempirical core pseudopotentials has been studied by comparing the results for atomic and molecular nitrogen obtained using contracted ANO basis sets with those obtained using both the primitive set and a segmented one. The primitive set has been optimized at the SCF level for atomic N treated as a five-electron pseudo-atom, and consists of 7s and 7p primitive GTOs supplemented by 2d and 1f GTOs optimized at the CI level. From this primitive set three contracted [3s 3p 2d 1f] sets have been obtained. The first one has been derived from the ANOs of the neutral atom, the second has been obtained from an averaged density matrix and the third one is a segmented set. For the atom, the segmented set gives a zero contraction error at the SCF level as it must be in valence-only calculations. The ANO basis sets show some small contraction error at the SCF level but perform better in CI calculations. However, for the diatomic N2 molecule the ANO basis sets exhibit a rather large contraction error in the calculated SCF energy. A detailed analysis of the origin of this error is reported, which shows that the conventional strategy used to derive ANO basis sets does not work very well when pseudopotentials are involved.

We calculated the optical absorption and luminescence energies of electrons trapped at oxygen vacancies in CaO using a consistent embedded cluster method which accounts for the long-range polarization effects and partial covalence of CaO. Optical absorption and luminescence energies of neutral 􏰏F center􏰎 and positively charged 􏰏F+ center􏰎 vacancies are calculated by means of time dependent density functional theory using the B3LYP exchange-correlation density functional. Our results demonstrate that using large basis sets to describe a diffuse nature of excited states, and properly accounting for long-range polarization induced by charged and excited defect states, is crucial for accurate predictions of optical excitation and luminescence energies of these defects.

High-resolution photoemission and first-principles density-functional calculations were used to study the interaction of oxygen with ZrC(001) and VC(001) surfaces. Atomic oxygen is present on the carbide substrates after small doses of O 2 at room temperature. At 500 K , the oxidation of the surfaces is fast and clear features for Zr O x or V O x are seen in the O ( 1 s ) , Zr ( 3 d ) , and V ( 2 p 3 ∕ 2 ) core levels spectra, with an increase in the metal/carbon ratio of the samples. A big positive shift ( 1.3 – 1.6 eV ) was detected for the C 1 s core level in O ∕ Zr C ( 001 ) , indicating the existence of strong O ↔ C or C ↔ C interactions. A phenomenon corroborated by the results of first-principles calculations, which show a CZrZr hollow as the most stable site for the adsorption of O. Furthermore, the calculations also show that a C ↔ O exchange is exothermic on ZrC(001), and the displaced C atoms bond to CZrZr sites. In the O ∕ Zr C ( 001 ) interface, the surface C atoms play a major role in determining the behavior of the system. In contrast, the adsorption of oxygen induces very minor changes in the C ( 1 s ) spectrum of VC(001). The O ↔ V interactions are stronger than the O ↔ Zr interactions, and O ↔ C interactions do not play a dominant role in the O ∕ V C ( 001 ) interface. In this system, C ↔ O exchange is endothermic. VC(001) has a larger density of metal d states near the Fermi level than ZrC(001), but the rate of oxidation of VC(001) is slower. Therefore the O ∕ Zr C ( 001 ) and O ∕ V C ( 001 ) systems illustrate two different types of pathways for the oxidation of carbide surfaces.

The vertical transitions of Cu atoms, dimers, and tetramers deposited on the MgO surface have been investigated by means of ab initio calculations based either on complete active space second-order perturbation theory or on time-dependent density functional theory. Three adsorption sites have been considered as representative of the complexity of the MgO surface: regular sites at flat (100) terraces, extended defects such as monoatomic steps, and point defects such as neutral oxygen vacancies (F or color centers). The optical properties of the supported Cu clusters have been compared with those of the corresponding gas-phase units. Upon deposition a substantial modification of the energy levels of the supported cluster is induced by the Pauli repulsion with the substrate. This causes shifts in the optical transitions going from free to supported clusters. The changes in cluster geometry induced by the substrate have a much smaller effect on the optical absorption bands. On F centers the presence of filled impurity levels in the band gap of MgO results in a strong mixing with the empty levels of the Cu atoms and clusters with consequent deep changes in the optical properties of the color centers. The results allow to interpret electron energy loss spectra of Cu atoms deposited on MgO thin films. (C) 2004 American Institute of Physics.

Aggregates of oxygen vacancies (F centers) represent a particular form of point defects in ionic crystals. In this study we have considered the combination of two oxygen vacancies, the M center, in the bulk and on the surface of MgO by means of cluster model calculations. Both neutral and charged forms of the defect M and ${M}^{+}$ have been taken into account. The ground state of the M center is characterized by the presence of two doubly occupied impurity levels in the gap of the material; in ${M}^{+}$ centers the highest level is singly occupied. For the ground-state properties we used a gradient corrected density functional theory approach. The dipole-allowed singlet-to-singlet and doublet-to-doublet electronic transitions have been determined by means of explicitly correlated multireference second-order perturbation theory calculations. These have been compared with optical transitions determined with the time-dependent density functional theory formalism. The results show that bulk M and ${M}^{+}$ centers give rise to intense absorptions at about 4.4 and 4.0 eV, respectively. Another less intense transition at 1.3 eV has also been found for the ${M}^{+}$ center. On the surface the transitions occur at 1.6 eV ${(M}^{+})$ and 2 eV (M). The results are compared with recently reported electron energy loss spectroscopy spectra on MgO thin films.

In this paper we address the study of optical and magnetic properties of metal oxides based on the cluster model approach and ab initio quantum chemistry techniques. Two different multireference approaches are used to compute the wave functions of the ground and the excited electronic states involved in optical transitions and magnetic interactions. The Difference Dedicated Configuration Interaction (DDCI) approach is a variational method based on the direct computation of energy differences between states. The Complete Active Space Self-Consistent Field (CASSCF) followed by Complete Active Space Second-Order Perturbation Theory (CASPT2) approach combines the Configuration Interaction (CI) techniques and many-body perturbation theory to account for electron correlation effects. Both methods turn out to be very suitable to estimate the differential electronic correlation and compute excitation energies. To illustrate the suitability of both methods, four applications are presented in this work: the study of d-d transitions and magnetic interactions in fcc transitionmetal oxides, and the study of excitonic and vacancy states in MgO.

The weakly avoided crossing between the two lowest 1Σ+ electronic states of a series of alkali halide molecules has been studied by means of the recently reported multistate complete active space second-order perturbation theory, MS-CASPT2, method. For a large enough basis set and a complete active space self-consistent field that includes part of the radial and angular correlation of the outermost halide electrons, the calculated crossing distance is in very good agreement with that predicted from the Rittner empirical potential. The study of the relevant parameters corresponding to the crossing region on these molecules has been extended to include the effect of a uniform electric field and a generalization of the empirical Rittner formula that includes the electric field effects is presented. The predictions made by the MS-CASPT2 method are also in agreement with those derived from the generalized Rittner potential. Finally, the possible implications of the present work on electron transfer processes at metal electrodes are discussed.

The optical absorption spectrum and lowest photoluminescence (PL) signal for silanone terminated silica nanostructures are studied using time-dependent density functional theory calculations on a range of realistic low energy silica nanocluster models. We show that the broad experimental absorption spectrum for silanone centers [V. A. Radtsig and I. M. Senchenya Russ. Chem. Bull. 45, 1849 (1996)] is most likely the result of a synergetic combination of inhomogeneous broadening, thermal broadening and the small energy differences between different excitations. We further demonstrate that upon relaxation of the excited state the excited electron and hole localize on only one silanone center, and that there is a clear and distinct link between the local environment of a silanone center and its absorption and PL spectra. Finally, we provide strong evidence that the silanone center does not have a double bond between the constituent silicon and oxygen atoms but rather can be probably more aptly described as the =Si(+)-O(-) charge-transfer species.

The character of low‐lying excited states of MgO has been examined using cluster models of the MgO crystal. Several different clusters were used; in general, the clusters contained a central Mg atom, one or more shells of nearest‐neighbor Mg and O atoms, and a large number of point charges to represent the Madelung potential in the space of the atoms explicitly included in the cluster. The excited orbital was occupied in two ways. First, an electron was added to a low lying excited level while the Mg2+ and O2− anion levels were kept fully occupied. In the second way, an electron was excited from the oxygen 2p band into a low lying excited level. The excited orbitals, for both of these cases are very similar. Although the energetics of the excited states depended on the cluster model, in particular whether it was Mg or O terminated, the general character of the excited orbital remained the same. In all clusters, the excited state orbital was diffuse and delocalized. The excited orbital has no simple or direct relationship to the 3s orbital of Mg2+. It is best described as a ‘‘conduction band’’ orbital. It extends up to and beyond the edge of the atoms explicitly included in the cluster. In our work, we did not find any evidence for localized excitonic states.

The analysis of ab initio cluster model wavefunctions shows that the bonding of CO on Pt(111) is of covalent nature being well described by the Blyholder mechanism. However, the NO Cu(111) interaction is found to be dominantly ionic. This ionic nature of the bond has important consequences as an avoided crossing between two electronic states of ionic and neutral character. The existence of this avoided crossing interaction indicates that the process is non-adiabatic. While this is a well known phenomenon in molecular physics, to the authors' best knowledge, this is the first time it is shown to happen in a gassurface reaction.