The role of nonadditive effects in the first solvation shell of Na+ and Mg2+ in liquid ammonia: Monte Carlo studies including three-body corrections

The solvation structure of Na+ and Mg2+ in liquid ammonia has been investigated using the Metropolis Monte Carlo scheme. Simulations have been carried out for four systems namely, one Na+ in 201 NH3 and one Mg2+ in 201 NH3 with and without three-body corrections. With the experimental density of pure liquid ammonia at 277 K and 1 atm of 0.9893 g cm(-3), a periodic cubic volume of side length 20.99 Angstrom was yielded. The Na+-NH3 and Mg2+-NH3 pair potentials as well as the corresponding three-body correction functions have been newly developed based on ab initio calculations with a basis set of double zeta quality. The results indicate the failure of the pair approximation in predicting structural properties of both systems: the three-body corrections reduce the first shell coordination number of Na+ and Mg2+ in liquid ammonia from 9 to 8 and from 10 (two of them lying in small peak beyond the first maximum) to 6, respectively. (C) 1997 American Institute of Physics.