Browsing by Subject "Intermolecular forces"
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Item Collision-induced absorption and anisotropy of the intermolecular potential(2002-05) Gustafsson, Magnus Sven; Frommhold, LotharA scheme is developed for quantum mechanical calculations of binary collisioninduced spectra which permits full inclusion of rotovibrational molecular degrees of freedom. A close-coupling scheme which includes the radiation in the Hamiltonian is used. The collision-induced absorption spectra of interacting atom–diatom and diatom–diatom pairs are investigated. The inclusion of the anisotropy of the intermolecular potential introduces couplings among the rotational levels of the diatomic molecules. Previous calculations of collision-induced spectra have almost exclusively been done using the isotropic potential approximation and we present an extensive investigation of the validity of that approximation. Absorption spectra in the rotational and fundamental bands of H2, induced by collisions with He, H, Ar, and H2 are calculated for various temperatures. In all of these, except for H2–H, the anisotropy of the intermolecular potential affect the absorption by 5-10% in certain parts of the spectra. Comparisons with the available measurements show very good agreement of the shapes of the spectral profiles, although the absolute intensities differ by up to 10% in some cases. These remaining differences between theory and measurements appear to be random and are generally smaller than the differences among comparable measurements. In the H2–H spectra the effect of the anisotropy of the potential turns out to be almost negligible at the temperature for which a full coupled quantum calculation was done. This is supported by spectral moment calculations. The smallness of the effect is believed to stem from the short range character of the anisotropic potential components for H2–H. Collision-induced absorption spectra of gaseous mixtures of deuterium hydride and helium in the rotational and fundamental bands of HD are calculated at a temperature of 77 K. The computed absorption profile agree with a measurement taken in the HD fundamental band. We also consider the interference phenomena of the HD permanent dipole with the HD–He interaction-induced dipole by computing the wings of various R(j) lines and of the P1(1) line in the single, binary collision limit. Agreement between theory and measurements is observed in the low-helium-density limit of the measured absorption line shapes.Item Investigations of diffuse intermolecular electronic systems(Texas Tech University, 1992-05) Muguet, Francis F.Diffuse intermolecular electronic systems, such as the hydrated electron or the immonia and water dimers, present both a theoretical and a practical computational challenge. The hydrated electron was discovered more than 25 years ago, yet there is still no consensus on an explanation of this phenomenon. A novel model is presented here whereby the hydrated electron consists in an itinerant dihydronium radical structure. Although electrostatically neutral, the itinerant radical is shown to behave as a negative charge carrier under the influence of an electric field. Within this perspective, the hydrated electron may be considered a quasiparticle. Contrary of the absence of agreement between many experiments and the old but still popular cavity model description, the energetics in the new model, are shown to be consistent with photophysical experimental data. In order to understand negatively charged water clusters, it is also proposed that a metastable bifurcated water dimer structure is able to bind an extra electron. Prior to our studies, no ab initio computations had been able to reproduce the experimental geometry of the ammonia dimer nor to predict a water dimer anion with Franck-Conden factors agreeing with those recently found in molecular beam experiments. In both cases the potential energy surface is determined by attractors corresponding to nonlinear and linear hydrogen bonded geometries, respectively. One attracter receives an unfair advantage in the computational procedure mainly because of the basis set superposition error (ESSE). There is still no agreement on a scheme for correcting the ESSE. A widely employed error estimation method is the counterpoise correction. A completely different new method is proposed using reerthonermalizatien of purified localized molecular orbitals. In terms of a ESSE corrected potential energy surface of the water dimer, a multi-attractor model of water is very briefly discussed. For further water molecular dynamics studies, we offer a new algorithm which we have developed specifically for a massively parallel computer.