ZINC(II) IN LIQUID-AMMONIA - INTERMOLECULAR POTENTIAL INCLUDING 3-BODY TERMS AND MONTE-CARLO SIMULATION

The failure of pair potentials in computer simulations in giving the correct coordination number for Zn(II) in liquid ammonia is investigated. Ab initio molecular orbital calculations for Zn(II)-(NH3)n complexes with n = 1,2,3,4, and 6 have been performed. Pair potential and three-body correction function for Zn(II)/ammonia were constructed from the Zn(II)-NH3 potential surface and more than 1500 configurations of the Zn(II)-(NH3)2 complex. Monte Carlo simulations were performed with these functions for a single Zn(II) ion in liquid ammonia. Inclusion of the three-body effects leads to a lower coordination number and a shift of the first peak from 2.10 to 2.22 angstrom. A small peak containing three ammonia molecules, observed beyond the first solvation sphere in the simulation with conventional pair potential, disappears and the first solvation shell of Zn(II) is thus reduced to exactly six ammonia molecules.