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Geometry optimizations, rearrangement mechanisms, reaction pathways, spectral intensities, and tunneling splittings are reported for the water trimmer at various levels of theory. Three of the four pathways which exist for a simple empirical rigid-molecule effective pair potential appear to be quite realistic, as evidenced by ab initio calculations of increasing quality up to DZP basis sets with correlations from second-order Moller-Plesset theory. The spectroscopic consequences of the nonrigidity implied by the various rearrangement mechanisms are also considered; the pathways permit the interconversion within two subsets of the 96 permutationally distinct (H2O)3 clusters that arise when only O-H hydrogen bonds, as opposed to chemical bonds, may break. The associated tunneling spectra have also been calculated within a Huckel-type approximation, including estimates of the splittings associated with the different mechanisms. Quartet splittings are associated with both of the rearrangements that are likely to produce experimentally observable effects, and the order of magnitude predicted for the smaller of these is in reasonable agreement with experiment.
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