### Abstract

The two‐body Hartree‐Fock potential for the ion‐water interaction and the two‐body Hartree‐Fock potential for the water‐water interaction have been used in the pairwise additivity approximation to study the Li^{+}(H_{2}O)_{ n }, the Na^{+}(H_{2}O)_{ n }, the K^{+}(H_{2}O)_{ n }, the F^{−}(H_{2}O)_{ n }, and the Cl^{−}(H_{2}O)_{ n } complexes, where *n* =2,3,4, ⋯ ,10. The complex configurations have been constrained to have either symmetrical geometries around the central ion or to be free to assume the lowest energy configuration. For *n* smaller than 5 (depending on the specific ion in consideration), the symmetrical configuration is the lowest energy configuration. For higher values of *n*, some of the water molecules tend to form a second shell of solvated water around the ion. The configurational optimalization was carried out only at *T* =0°K; but for a small cluster containing only four molecules of water, calculations have been performed at *T* =298°K. From the study at 298°K we have computed the correlation functions*g* _{I–O}, *g* _{I–H}, *g* _{O–O}, *g* _{O–H}, and *g* _{H–H} (where the subscript I is a shorthand notation for ``ion''). Correlation functions are reported for the cluster F^{−}(H_{2}O)_{ n } at *T* =298°K with *n* =27. By comparing the results obtained at *T* =0°K and with *n* =10, with those obtained at *T* =298°K and *n* =4 and finally with the results obtained at *T* =298°K and *n* =27, we feel confident that the conclusions (given below) will remain valid for *n* >27 and at *t* ≠0°K. The coordination numbers for the ion‐water clusters are computed (approximatively) to be about 4 for Li^{+}, between 5 and 6 for Na^{+}, between 5 and 7 for K^{+}, between 4 and 6 for F, and between 6 and 7 for Cl^{−}. In the first solvation layer, the average ion oxygen distances are 1.9–2.0 Å for Li^{+}, 2.3–2.4 Å for Na^{+}, 2.8–2.9 Å for K^{+}, 2.7–2.8 Å for F^{−}, and 3.4–3.5 Å for Cl^{−}. The validity of the pairwise additivity approximation has been tested for the small clusters ion (H_{2}O)_{ n } with *n* =2,3 (and 4 and 5 for Li^{+} and F^{−}).

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