The Cu2+-NH3 pair potential and the corresponding three-body correction functions have been newly constructed on the basis of ab initio calculations of double-zeta quality. Monte Carlo simulation results for Cu(II) in liquid ammonia prove the failure of the pair potential approximation, even in the prediction of rough structural properties. Simulation including three-body potentials leads to a first-shell coordination number of 6 for Cu2+ in liquid ammonia at 240 K. The structure of the solvated ion is discussed in terms of radial distribution functions, coordination numbers, energies, and angular distributions.