X − ⋅(H 2 O) n=1–4  [X=F, Cl, Br, I] have been studied using high level ab initio calculations. This extensive work compares the structures of the different halide water clusters and has found that the predicted minimum energy geometries for different cluster are accompanied by several other structures close to these global minima. Hence the most highly populated structures can change depending on temperature due to the entropy effect. As the potential surfaces are flat, the wide-ranging zero point vibrational effects are important at 0 K, and not only a number of low-lying energy conformers but also large amplitude motions can be important in determining structures, energies, and spectra at finite temperatures. The binding energies, ionization potentials, charge-transfer-to-solvent (CTTS) energies, and the O–H stretching frequencies are reported, and compared with the experimental data available. A distinctive difference between F−⋅(H2O)n and X−⋅(H2O)n (X=Cl, Br, I) is noted, as the former tends to favor internal structures with negligible hydrogen bonding between water molecules, while the latter favors surface structures with significant hydrogen bonding between water molecules. These characteristics are well featured in their O–H spectra of the clusters. However, the spectra are forced to be very sensitive to the temperature, which explains some differences between different spectra. In case of F−⋅(H2O)n, a significant charge transfer is noted in the S0 ground state, which results in much less significant charge transfer in the S1 excited state compared with other hydrated halide clusters which show near full charge transfers in the S1 excited states. Finally, the nature of the stabilization interactions operative in these clusters has been explained in terms of many-body interaction energies.

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F − (H 2 O) n  (n=1–6) clusters have been studied using ab initio calculations. This is an extensive work to search for various low-lying energy conformers, for example, including 13 conformers for n=6. Our predicted enthalpies and free energies are in good agreement with experimental values. For n=4 and 6, both internal and surface structures are almost isoenergetic at 0 K, while internal structures are favored with increasing temperature due to the entropic effect. For n=5, the internal structure is favored at both 0 and 298 K under 1 atm. These are contrasted to the favored surface structures in other small aqua–halide complexes. The ionization potential, charge-transferto-solvent (CTTS) energy, and O–H stretching vibrational spectra are reported to facilitate future experimental work. Many-body interaction potential analyses are presented to help improve the potential functions used in molecular simulations. The higher order many-body interaction energies are found to be important to compare the energetics of the various conformers and compare the stability of the internal over the surface state.

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In spite of a spate of studies of various water clusters, a few theoretical studies on the water heptamer are available. State-of-the-art ab initio calculations are thus carried out on twelve possible water heptamer structures to explore the conformation as well as spectroscopic properties of this water cluster. Two three-dimensional cagelike structures comprised of seven-membered cyclic rings with three additional hydrogen bondings were found to be the lowest-lying energy heptamer conformers. The global minimum energy structure was found to be 0.5 kcal/mol lower than the other. The zero-point energy uncorrected and corrected binding energies of the global minimum energy structure are 55.2 and 37.9 kcal/mol, respectively. An almost two-dimensional ring conformer, which is only 1 kcal/mol above the global minimum at 0 K, could be more stable above 150 K. The vibrational spectra of different heptamer conformers are discussed and compared with the spectra of the hexamer and octamer water clusters.