Abstract: An empirical potential of binary molecular clusters of (NH₃)_m(H₂O)₄ is proposed.Stable model structures of NH₃(H₂O)₄ are optimized by using a fast molecular dynamics algorithm.The optimal structures are refined by using conventional ab initio optimization method.Optimization and frequency calculations are carried out at different levels of density functional theory (DFT/B3LYP) and Mller-Plesset second-order perturbation method(MP2) with 6-311++G** basis set.It is found that stable model structures remain the minima at B3LYP and MP2 levels.12 new ab initio stable isomers of NH₃(H₂O)₄,including the second and third ones are obtained.The results show that there is no inevitable relevancy between stability and dipole moment,and the number of hydrogen bonds of isomers.

Key words: ammonia water clusters, empirical potential, optimized structure, hydrogen bond, DFT