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ABSTRACT: Ab initio and model calculations are used to study weak interactions in clusters of small molecules. Correlation as well as basis set effects on electrical interaction and non-electrical interaction, as well as electrical response properties of monomers, are examined. Electrical response properties for argon and hydrogen sulfide are calculated. Two different approaches to basis set augmentation are compared--namely, augmentation with bond functions and with atom-centered functions. Bond functions are found to greatly increase the basis set superposition error (BSSE) at the correlated level and to seriously affect the location of potential minima. Diffuse polarization functions, on the other hand, improve the description of the most important components of weak bonding: electrical interaction and dispersion. Ab initio calculations are used to explore the surfaces of the water dimer, hydrogen sulfide dimer, and dimers of hydrogen sulfide with rare gases. The nature of the interactions in these dimers vary in the importance of electrical and dispersion interaction: in the first electrical interaction dominates, in the second dispersion dominates but electrical interaction is still significant, and in the third, dispersion dominates and electrical interaction plays a minimal role. Model potential surface of water-benzene (a dimer where electrical interaction is most important) is used to help identify free rotor state in the complex. Parameters are obtained for model calculations for clusters involving hydrogen sulfide. These clusters are compared with water clusters; there are certain structural similarities but significantly different energetics. Interconversion barriers are mostly smaller for H2 S complexes than for the corresponding water complexes, due to smaller electrical stabilization in H2 S complexes. Ab initio calculations have been carried out to generate a potential energy surface for the Ar-H2 S and Ne-H2 S weakly bonded clusters. Both surfaces display low energy troughs with small barriers for the "orbit" of the rare gas about H$S Basis set and correlation effects have been analyzed through a series of calculations at different levels, and the feature of a low energy trough seems assured. The nature of these surfaces has only little to do with electrostatic interaction, and so, a fairly good fit of the non-electrical part of the potential surface is obtained with atom-atom Lennard-Jones potential terms. With different surface fits and a fully anharmonic analysis of the ground vibrational state, on-average rotational constants were calculated via rigid-body diffusion quantum Monte Carlo. These calculations show that certain intriguing spectroscopic manifestations of deuterium substitution in the complex with argon are associated primarily with the trough feature of the surface. The unusual deuteration effects are not present in the neon complex. Using an Ar-H2 S interaction potential derived from ab initio calculations, rigid body diffusion quantum Monte Carlo calculations have been carried out to obtain structural and energetic information for clusters of H2 S with two to six argon atoms. The potential surfaces of each of the clusters exhibit multiple minima. The lowest energy structures show a preference for argons to surround H2 S outset of aggregation. Barriers between the lowest-lying minima tend to be small resulting in ground state rotational constants that differ from those of equilibrium structures by up to 100 MHz.
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