Car-Parrinello molecular dynamics simulations of Na–Cl ion pair in liquid water

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.