Systematic Study of Structural and Thermodynamic Properties of HCl(H2O)n Clusters from Semiempirical Replica Exchange Simulations

The structural and thermodynamic properties of HCl(H2O)(n) clusters with n = 4-10 are studied using Born-Oppenheimer replica exchange molecular dynamics simulations with the PM3-MAIS semiempirical Hamiltonian. Independently of the cluster size, the simulations predict that HCl exists in the dissociated form in all low-energy isomers. Different local structures are identified within the clusters due to the presence of the dissociated proton, including Zundel, Eigen, Eigen-like, H7O3+, and intermediate Zundel-Eigen configurations. As the cluster size increases, several groups of isomers are identified, whose relative stabilities vary as a function of temperature. A detailed analysis of the heat capacity indicates that the melting behavior of HCl(H2O)(n) clusters is strongly size-dependent. In particular, melting is observed in clusters with n = 7-10 in the temperature range T = 100-150 K. By contrast, melting is not observed in clusters with n = 4-6. Minimum energy structures for HCl(H2O)(n) clusters with n = 11-15 and n = 21 are also characterized.