Recent advances in electronic structure theory and their influence on the accuracy of ab initio potential energy surfaces

Recent advances in electronic structure theory and the availability of high speed vector processors have substantially increased the accuracy of ab initio potential energy surfaces. The recently developed atomic natural orbital approach for basis set contraction has reduced both the basis set incompleteness and superposition errors in molecular calculations. Furthermore, full CI calculations can often be used to calibrate a CASSCF/MRCI approach that quantitatively accounts for the valence correlation energy. These computational advances also provide a vehicle for systematically improving the calculations and for estimating the residual error in the calculations. Calculations on selected diatomic and triatomic systems will be used to illustrate the accuracy that currently can be achieved for molecular systems. In particular, the F+H2 yields HF+H potential energy hypersurface is used to illustrate the impact of these computational advances on the calculation of potential energy surfaces

Orbital interactions and chemical reactivity of metal particles and metal surfaces

A review is presented with 101 refs. on chem. bonding to metal surfaces and small metal particles demonstrating the power of symmetry concepts to predict changes in chem. bonding. Ab-initio calcns. of chemisorption to small particles, as well as semiempirical extended Hueckel calcns. applied to the study of the reactivity of metal slabs are reviewed. On small metal particles, classical notions of electron promotion and hybridization are found to apply. The surroundings of a metal atom (ligands in complexes, other metal atoms at surfaces), affect bonding and reactivity through the prehybridization they induce. A factor specific for large particles and surfaces is the required localization of electrons on the atoms involved in the metal surface bond. At the surface, the bond energy is found to relate to the grou8p orbital local d. of states at the Fermi level. The use of this concept is extensively discussed and illustrated for chemisorption of CO and dissocn. of NO on metal surfaces. A discussion is given of the current decompn. schemes of bond energies and related concepts (exchange (Pauli)-repulsion, polarization, charge transfer). The role of non-orthogonality of fragment orbitals and of kinetic and potential energy for Pauli repulsion and (orbital) polarization is analyzed. Numerous examples are discussed to demonstrate the impact of those concepts on chem. bonding theor

Theoretical modeling of infrared emission from neutral and charged polycyclic aromatic hydrocarbons. I.

Since the discovery of interstellar infrared emission features in the 3.3-12.7 mum wavelength range three decades ago, the carriers of these features have been the subject of much debate. Recent observational work with the Infrared Space Observatory, experimental work, and quantum chemical calculations concerning positively charged polycyclic aromatic hydrocarbon (PAH) molecules point to the infrared fluorescence of such species. This paper presents a model of the interstellar infrared emission between 3.3 and 12.7 mum from a population of symmetric, condensed polycyclic aromatic hydrocarbons composed of up to 54 carbon atoms. We describe the infrared emission intensity in terms of the size of the emitting molecule, its charge, and its temperature probability distribution function. The model takes the charge state (anion, neutral, cation of charge state up to +3) into account self-consistently, employing the most recent quantum chemically calculated infrared cross sections of such species. This paper provides an exploratory study to illustrate the dependence of the interstellar infrared emission on the polycyclic aromatic hydrocarbon charge distribution. We conclude that the charge state of the PAH has an important effect on the emitted infrared spectrum. The 3.3 mum stretching mode and, to a lesser extent, the 11-15 km C-H out-of-plane bending modes produce significant emission relative to the other infrared features and originate predominantly from neutral and anionic PAHs. The 6-8 mum emission from the C-C stretching modes, in contrast, originates mainly from charged PAHs with only a partial contribution from neutrals. For heavily ultraviolet irradiated regions such as the Orion Bar, multiply positively charged PAHs are the norm and contribute significantly in this wavelength region. However, because the total infrared emission is a sum over various charge states of different molecules, the ratios of the infrared emission bands do not vary much for G(o)/n(e) less than or equal to 10(3) cm(3). This range includes conditions relevant to both the diffuse interstellar medium and typical reflection nebulae. Larger variations in the interstellar infrared emission features can be expected from photodissociation regions associated with dense H II regions such as the Orion Bar (G(o)/n(e) similar to 10(4))

THE DISSOCIATION-ENERGIES OF ALH2 AND ALAR

The D-0 values for AlH2 and AlAr are computed using the coupled cluster approach in conjunction with large basis sets. Basis set superposition and spin-orbit effects are accounted for, as they are significant due to the small binding energy. The computed dissociation energy (D-0) for AlAr is 114 cm(-1), which is 93% of the experimental value (122.4 cm(-1)). Our best estimate for the Al-H-2 binding energy is 38+/-26 cm(-1)