Singlet–triplet splittings as obtained from the Xα-scattered wave method: A theoretical analysis

The theoretical determination of singlet–triplet splittings of the excited states of closed-shell molecules is discussed with emphasis on the use of the Xα–scattered wave method. With this method, splittings can be computed in two independent ways. Both will have corrections due to orbital relaxation effects. The singlet–triplet splittings obtained with the Xα–scattered wave method for several large organic molecules are reviewed. It is seen that the results using the two splitting formulas differ by a large amount; in fact, they differ by the magnitude of the computed splitting. We provide a criterion for choosing the results obtained with one method over the other by showing that they both give upper bounds to the best result obtainable within the framework of the model. However, because of the large differences between the two methods, the quantitative value of the splitting should be viewed with caution.     

Cluster-model study of the interaction of halogen atoms with Ag clusters

Analysis of ab initio wave functions shows that the interaction between halogen atoms (F, Cl, and Br) and Ag clusters is ionic, and that the halogen ionicity is essentially ?1. The interaction and the bonding arise, almost entirely, from the Coulomb attraction between the charged halogen and the metal and from polarization of the two subunits. Large shifts in equilibrium bond distances and vibrational energies are caused by an external electric field. These changes arise from a dominant Stark effect.     

Exchangelike effects for closed-shell adsorbates: interface dipole and work function

Based on detailed theoretical analyses, we present, for the first time, direct evidence that the significant interface dipole commonly observed for atoms and molecules physisorbed on metal surfaces originates from exchangelike effects. In the case of Xe, previously proposed contributions from chemical interactions do not play a significant role.     

Ultraviolet oscillator strengths for carbon,nitrogen and oxygen ions

Oscillator strengths were calculated for u.v. transitions in carbon, nitrogen and oxygen ions. Single and multiconfiguration numerical Hartree-Fock wavefunctions were used to represent initial and final states of the various transitions, and both dipole length and dipole velocity transition matrix elements were calculated. A careful choice of a few important configurations leads in most cases to multiconfiguration results which are in good agreement with experiment, and with other theoretical calculations.     

Local models for the interaction of adsorbates with surfaces

Local models on the basis of the surface cluster approach have been applied extensively in the study of surface and adsorbate systems. The energetic details of the adsorbate-substrate interaction can be characterized in these models by the constrained space orbital variation (CSOV) method. This method allows a detailed study of adsorbate binding in different surface sites. Further, consequences of the chemisorptive interaction for adsorbate valence photoemission and vibrational properties of adsorbed molecules can be studied.     

Core level shake up structures of N2 adsorbed on nickel surfaces: Cluster models

Self-consistent Hartee-Fock-LCAO calculations on a linear NiN₂ cluster are used to explain the two-peak structure observed in core level photoemission from N₂ adsorbed on the Ni (100) surface. The two observed peaks are due to different N₂ core hole final states, a screened and an unscreened final state that can be clearly identified in the cluster model. The energy separation and intensity ratio (computed using the sudden approximation) of the two final states in NiN₂ are comparable with the experimental peak separation and intensity ratio for reasonable cluster geometries. A comparison with previous model studies on NiCO is used to explain the different shape of the core level spectra of N₂ and CO adsorbed on nickel surfaces.     

On the nature of the low-lying states of TiO

A series of CAS SCF calculations are used to describe the lowest states of TiO. TiO is described as having polarized double bonds involving the Ti 3d orbitals. The nature of the excited states is discussed.     

Core binding energies for clusters deposited on different insulating substrates: esca spectra and theoretical electronic structure studies

The 4f core-level X-ray photoemission of small Pt clusters deposited on Teflon and SiO₂ substrates has been investigated. The experimental results are discussed and related to other data reported in the literature. Parallel results are presented for ab initio selfconsistent field, SCF, calculations of the core ionization potentials, IPs, of small Li clusters. These model calculations provide qualitative information about the nature and magnitude of the initial and final state contributions to the IPs for small clusters. We find that the binding energy shifts and linewidth broadening depend primarily on the cluster size and on the cluster substrate interaction. They do not result from electrostatic effects of the unit positive charge remaining on the ionized cluster. Our calculations show that this Coulomb effect does not apply to very small clusters.