With the high accuracy afforded by the sextuple correlation consistent basis set of Dunning, we have calculated energy levels, dissociation energies, equilibrium distances, and other spectroscopic constants for eleven valence and four Rydberg states of the CH radical. Comparisons with experimental and previous theoretical results are made for each state that has been treated. An understanding of their binding is attempted by means of simple valence bond-Lewis diagrams.

We have studied the interaction of an oxygen molecule with Al clusters and Al(111) using both wave-functionbased quantum chemistry methods and density functional theory (DFT). These calculations were motivated by the fact that molecular beam experiments indicate that the adsorption of O 2 on Al(111) should be activated whereas periodic DFT calculations yield purely attractive adsorption paths for almost all impact configurations of O 2 on Al(111). On small Al 4 clusters, accurate wave-function-based quantum chemistry methods find a non-vanishing barrier in the O 2 adsorption. The DFT calculations for slabs and larger Al clusters confirm the important role of spin effects for the O 2 dissociation barrier on Al. The results indicate that exchange-correlation effects play a crucial role for the determination of the adsorption barrier in the O 2 /Al system but their determination is hampered by serious technical problems that are discussed in detail.

Employing both multireference configuration interaction (MRCI) and density functional theory (DFT) methods, we have studied the interaction of O₂ with a tetrahedral Al₄ cluster in the total spin triplet state. For a parallel to the base approach of O₂ facing an apex of the pyramid, the O₂ adsorption is hindered by a barrier. Both the MRCI and the DFT calculations show that after a small barrier, there are two local energy minima: a shallow one just above the apex atom and another deeper one below the apex atom. The latter corresponds to dissociative O₂ adsorption. We discuss the implications of these findings for the understanding of O₂ adsorption on defect sites of Al surfaces.

The aim of this research is to show that the processes of absorption chargeexchange and photo-association in A + B + collisions together with the processes of AB + photo-dissociation in the case of strongly non-symmetric ion-atom systems, significantly influence the opacity of stellar atmospheres in ultraviolet (UV) and extreme UV (EUV) region. In this work, the significance of such processes for solar atmosphere is studied. In the case of the solar atmosphere the absorption processes with A = H and B = Mg and Si are treated as dominant ones, but the cases A = H and B = Al and A = He and B = H are also taken into consideration. The choice of just these species is caused by the fact that, of the species relevant for the used solar-atmosphere model, it was only for them that we could determine the necessary characteristics of the corresponding molecular ions, i.e. the molecular potential curves and dipole matrix elements. It is shown that the efficiency of the examined non-symmetric processes within the rather wide corresponding quasi-molecular absorption bands in the far-UV and EUV regions is comparable and sometimes even greater than the intensity of the known symmetric ion-atom absorption processes, which are included now in the models of the solar atmosphere. Consequently, the presented results suggest that the non-symmetric ion-atom absorption processes also have to be included ab initio in the corresponding models of the stellar atmospheres.