Clusters of hydroxide ion, HO-(H2O)(n=1-4), have been studied by high level ab initio calculations in order to better understand the first coordination shell of OH- ions. Geometry optimizations were performed at Hartree-Fock, density functional theory and second order Moller-Plesset perturbation theory levels using the 6-31+G(d,p) basis set. Single point energy calculations were carried out on the optimized geometries using the more extended 6-311+G(2df,2p) basis set and a higher level of electron correlation, namely fourth-order Moller-Plesset perturbation theory. For the n=1-3 clusters, only structures with the hydroxide ion hydrogen bonded to all waters molecules were considered. For the n=4 cluster, three minima were found; the most stable species has all four waters directly bound to the hydroxide ion, while the other two clusters have only three waters in the first coordination shell. In addition, the transition state connecting the cluster containing four waters in the first coordination shell to the species having three waters in the coordination shell was characterized. The barrier for this rearrangement is very low (1.82 kcal/mol), and we predict this process to occur on the picosecond time scale. The thermodynamic properties (enthalpy, entropy and Gibbs free energy) for the formation of the clusters have been calculated for all the species (including the fully deuterated clusters). Comparison of our calculations with experimental data reveals good agreement in the free energy. Nevertheless, our ab initio results suggest that for the n > 1 clusters, both -Delta H-0 and -Delta S-0 are larger than those reported from experiment and new experiments may be necessary to obtain accurate experimental values. (C) 2000 American Institute of Physics. [S0021-9606(00)30909-6].