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In this paper we present a quantum-mechanical study of anions in water clusters, X-(H2O)n (X=Cl, Br, I, and n=1–6). Molecular orbital calculations at the self-consistent field (SCF) level and at the second-order Mo\ller–Plesset (MP2) level were performed using extended basis sets. Full structural optimization was conducted at the MP2 level for n=1 and at the SCF level for n=2–6. The energies and charge distribution of X-(H2O) were calculated at the MP2 level, while the energies of the X-(H2O)n (n=2–6) clusters were calculated at the MP2 level using the SCF optimized geometry. Calculations of total and sequential enthalpies of hydration and for the vertical ionization potentials were conducted for X-(H2O), the hydrogen bonded and linear isomers of X-(H2O)2, the pyramidal structure of X-(H2O)3, and the interior and surface isomers of X-(H2O)n, n=4–6. The calculated hydration enthalpies account well for their experimental size dependence for n=1–6. However, the isomer specificity of the hydration enthalpies is reflected by a small energy difference (δ=1–5 kcal mol-1) between the surface and interior isomers at a fixed n, precluding the assignment of structural isomers on the basis of ground-state energetics. The cluster size dependence and isomer specificity of the calculated vertical ionization potentials in conjunction with experimental data provide a diagnostic tool for the structural assignment of isomers and for the distinction between surface and interior structures. The central prediction emerging from the structure-energetic relations based on cluster size dependence and isomer specificity of vertical ionization potentials, is the prevalence of surface structures for Cl-(H2O)n (n=2–6), Br-(H2O)n (n=2–6), and I-(H2O)n (n=2–5), while a ‘‘transition’’ from surface to interior structure may be exhibited for I-(H2O)6. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations and ab initio calculations, at the Hartree–Fock level and Mo\ller–Plesset perturbation to the second order energy, have been performed to investigate the modeling of propane using nonlinear three-site pair potentials. Simulation results for the structure and thermodynamic properties of saturated liquid propane compare well with the experimental data when potential models are used in which the interaction sites are located at or close to the geometric centers of CH3 and CH2 groups. One of the empirical potentials has also been compared with the ab initio results for different molecular orientations. The three-site model favors stacked orientations whereas the ab initio propane dimer favors T orientation. This difference is due to the planar geometry of the model molecules. The effect of attractive forces on the structure and thermodynamic properties in the simulated liquid has also been studied and found to be small. [ABSTRACT FROM AUTHOR]