Theoretical Study of Microscopic Solvation of Ammonia in Water Clusters:  NH₃(H₂O)_n, n = 3, 4

A number of significant structures of NH₃(H₂O)_n (n = 3, 4) clusters have been identified by ab initio Monte Carlo simulated annealing, a procedure that efficiently samples minima on a potential energy surface. In this procedure, energies were computed ab initio at each Monte Carlo step by the B3-LYP density functional method with the 6-31G* basis set. All geometries of the isomers found for each cluster were refined in full conventional geometry optimizations, and frequency analyses were performed at both the B3-LYP and MP2 levels with the 6-311+G(d,p) basis set. The B3-LYP and MP2 energy orders were confirmed with single point QCISD(T) calculations with the 6-311+G(d,p) basis set performed on the MP2 optimized geometries. Only associated isomers were found for NH₃(H₂O)₃. However, for NH₃(H₂O)₄ both associated and dissociated structures were found.