Recent experimental results (Hoeft et al 2001 Phys. Rev. Lett. 87 086101) have questioned the capability of current theoretical methods for describing the bonding of NH3, CO, and NO with the NiO(100) surface. We show that these systems do indeed represent a challenge to theory. For different reasons, density functional theory (DFT) fails in describing the bonding of these molecules to the NiO surface. The gradient-corrected functionals which work better for the properties of NH3/NiO and CO/NiO (energies, geometries, vibrations) provide wrong answers for NO/NiO and vice versa. This is not due to the well-known difficulty as regards DFT describing the insulating character of NiO. In fact, exactly the same problem is found for isolated Ni2+ impurities in MgO. A correct description of the bonding of both closed-shell (NH3 and CO) and open-shell (NO) molecules to NixMg1−xO is obtained only after inclusion of dynamical correlation and dispersion forces via wavefunction-based methods. However, even with correlated calculations some uncertainties exist regarding the predicted value of the energy of adsorption of NO on NiO. While CASPT2 calculations reach reasonable agreement with experiment, the results of approximate coupled-cluster calculations (the multi-configuration coupled-electron-pair approach) substantially underestimate the adsorption energy.

Aggregates of oxygen vacancies ͑F centers͒ 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 ϩ 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 ϩ centers the highest level is singly occupied.For the ground-state properties we used a gradient corrected density functional theory approach. The dipoleallowed 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 ϩ 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 ϩ center. On the surface the transitions occur at 1.6 eV (M ϩ ) and 2 eV ͑M͒. The results are compared with recently reported electron energy loss spectroscopy spectra on MgO thin films.

The NO/NiO(100) system represents an excellent test case for the theory of surface chemical bond since accurate information about geometry, adsorption strength, and spin properties is available from experiments performed on NiO and Ni-doped MgO powders, single crystals, and thin films. We used cluster models to describe the NO/NiO interaction in combination with density functional theory (DFT) and wave function-based methods. We have identified four major aspects of the interaction: (1) the bonding cannot be described by a single determinant; (2) a spin-polarized DF-B3LYP approach gives reasonable adsorption properties at the price of a physically incorrect spin distribution; (3) a key ingredient of the interaction is the Coulomb repulsion within the Ni 3d shell; since this term is described very differently depending on the exchange-correlation functional it can result in overbound generalized gradient approach or Becke, Lee, Yang, and Parr or in strongly unbound (HFLYP) systems depending on the DFT approach; (4) the proper inclusion of the dynamical correlation is essential to treat the on-site Coulomb repulsion within the Ni 3d shell and to provide an accurate bond strength. In fact, the explicitly correlated complete-active-space second-order perturbation theory method gives results in overall agreement with the experiment. This shows the importance of treating on the same footing spin and electron correlation as well as the multiconfiguration character of the wave function.

The weakly avoided crossing between the two lowest 1 ⌺ ϩ 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.

A theoretical model is presented aimed to provide a detailed microscopic description of the electron transfer reaction in an electrochemical environment. The present approach is based on the well-known two state model extended by the novelty that the energy of the two states involved in the electron transfer reaction is computed quantum mechanically as a function of the solvent coordinate, as defined in the Marcus theory, and of the intensity of an external electric field. The solvent conformations defining the reaction coordinate are obtained from classical molecular dynamics and then transferred to the quantum mechanical model. The overall approach has been applied to the electron transfer between a chloride anion and a single crystal Cu(100) electrode. It is found that the solvent exerts a strong influence on the equilibrium geometry of the halide and hence on the relative energy of the two states involved in the electron transfer reaction. Finally, both solvent fluctuations and external field facilitate the electron transfer although solvent effects have a stronger influence.