Title: Car-Parrinello molecular dynamics simulations of Na⁺-Cl^- ion pair in liquid water
Authors: J.M.Khalack (Division of Physical Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden; Bogolyubov Institute for Theoretical Physics of the National Academy of Sciences of Ukraine, 14b Metrologichna Str., 03143 Kyiv, Ukraine) , A.P.Lyubartsev (Division of Physical Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden)

The aqueous solvation shell of a Na⁺-Cl^- pair is studied using Car-Parrinello molecular dynamics simulations. Water-mediated and contact states of the ion pair are investigated. The first hydration shell of the Na⁺ ion is found to be octahedral with one vacant position for both states. In the contact state one of the water molecules is substituted by the Cl^- ion. The first hydration shell of the Cl^- is less structured and strongly effected by the proximity of the Na⁺ in the contact state. The oxygen coordination numbers for Na⁺ and Cl^- are 4.9 and 5.6 for the water-mediated state, and 3.6 and 6.4 for the contact state. The corresponding hydrogen coordination numbers are 10.9 and 5.2; 9.2 and 3.9, respectively.

Key words: ab initio molecular dynamics, hydration, Sodium ion, Chloride ion
PACS: 61.20.Ja