Halogen dioxides (FOO, ClOO, BrOO, OClO, OBrO), their cationic and anionic derivatives and two isomers of ClO3 have been studied by means of density-functional theory (DFT) and the results compared with those from high level ab initio molecular orbital calculations. Three different density functionals (SVWN, B3LYP, and G96LYP) combined with a 6-311+G(2df ) basis set were used to obtain geometries and vibrational frequencies, which were then compared with MP2 (second-order Moller–Plesset), QCISD, and CCSD(T) (coupled-cluster single double triple) results. The B3LYP/6-311+G(2df ) calculations generally give geometries and frequencies in excellent agreement with those calculated from high level ab initio calculations such as CCSD(T). Exceptions, such as ClOO and BrOO, arise when high spin contamination at B3LYP level produces spurious results. Atomisation enthalpies evaluated at B3LYP/6-311+G(3df ) level of theory are observed to be in good agreement with the experimental values. In some particular cases this agreement is better than that obtained at CCSD(T)/6-311+G(3df ) level. For ionization enthalpies the CCSD(T) calculations seem to be superior to the DFT ones. Wave function instabilities [with respect to the UHF (unrestricted Hartree–Fock) transformation in the case of the cations and internal symmetry breaking in the case of the OXO (X=Cl, Br) compounds and the C3v isomer of ClO3] are observed less frequently when DFT methods are used.
The water hexamer has been studied with a classical water-water interaction potential and by quantum calculation at both RHF and MP2 levels. The influence of a virtual metal surface on (H 2 O) 6 has been modeled through geometry constraints on the cluster. Additional data on (H 2 O) 2 and (H 2 O) 3 are presented to assist the interpretation of the results obtained for the hexamer. These calculations suggest that water molecules in the first layer with their hydrogens pointing away from the surface ('flip up') only occur for a small range of values of surface lattice constants. In all other cases, the dipole moment of the water molecules is found to lie nearly parallel to the metal surface.
The broadening and shift of the astrophysically important resonance lines of singly ionised calcium and magnesium due to collisions with atomic hydrogen were calculated. Adiabatic molecular potentials for CaH+ and MgH+ were obtained from ab initio molecular structure programs, assuming frozen cores. Reasonable agreement was obtained with published calculations for these potentials, where available. The collision dynamics were treated semiclassically and broadening rates evaluated over a temperature range 500-5000 K. The results for broadening were larger than other theoretical estimates, based on long-range forces, but agree well with the values derived from analyses of solar profiles. Difficulties of correlating the broadening of a given spectral line by helium with that produced by atomic hydrogen are discussed. Values for the shifts and the fine-structure changing rates are also reported.
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