An intermolecular potential function for Zn(n)/H2O has been constructed on the basis of an ab initio (DZP-ECP) calculated energy surface. Due to the failure of the conventional pair potential ๒ predicting the correct hydration structure of Zn(II), a correction for non-additive terms had to be made; for this purpose, a new algorithm ("Nearest Neighbour Ligand Correction") was designed, which is based on ab initio calculations of metal ion monohydrate interaction with a second ligand (water or anion). The correction terms derived from this algorithm depend on the positions of other ligands present in the first coordination shell of the metal ion and are added to the pair potential energy. With the help of this feature, the correct hydration number for Zn(II) resulted in a Monte Carlo simulation for infinitely dilute solution. Monte Carlo simulations could be performed therefore, also for ZnCl2 solutions of varying concentration. The results of these simulations are in good agreement with experimental structural data and they reveil numerous details about microstructure and species distribution: the hydration shell of Zn(n) is dominantly of octahedral symmetry up to 3 molal concentration; in 5 molal solution, a breakdown of this structure is observed. In 1 mold solution, contact ion pairs form only to a marginal extent, whereas in 3 M and 5 M solutions the species ZnCl+(H2O)n and ZnCl2(H2O)m (chlorides in trans-positions) are of considerable importance. Besides these contact ion pairs, solvent separated ion pairs (outer-sphere complexes) are present at all concentrations. Ligand distribution plots have been used to illustrate the main structural entities, and the simulation results have been compared to data for Zn(II)/chloride complex formation from various experimental methods.
