Ab Initio Monte Carlo Simulated Annealing Study of HCl(H₂O)n (n = 3, 4) Clusters

Low-lying structures of HCl (H 2 O) n ( n = 3, 4) clusters have been studied by ab initio Monte Carlo simulated annealing (MCSA), a procedure which efficiently samples minima on a potential energy surface. In the Monte Carlo simulated annealing procedure, energies were computed ab initio at each Monte Carlo step by the B3-LYP density functional method with 6-31G* basis sets. 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 6-311+G** basis sets. The stability of the B3-LYP and MP2 energy orderings was tested in single point QCISD(T) calculations performed at the MP2 optimized geometries. Only isomers with strong H−Cl interaction were found for HCl(H 2 O) 3 . However, both associated- and dissociated-HCl structures of HCl(H 2 O) 4 were found.