Hydration characteristics of Ca2+ and Mg2+: a density functional theory, polarized continuum model and molecular dynamics investigation

In this work, density functional theory, M?ller?Plesset second-order perturbation theory, and ab initio molecular dynamics (AIMD) were used to investigate hydrated characteristics of Mg2+ and Ca2+ as a function of coordination number in the first hydration shell (CN) and cluster size. It is generally accepted that the CNs of Mg2+ and Ca2+ are both six. Calculations show that the hydration of Mg2+ generally prefers six-coordinated structures, whereas the CN value of Ca2+ varies from 6 to 8 as the hydration proceeds. Moreover, the first hydration of Ca2+ is found to be more flexible than that of Mg2+, as indicated by the results of transition state calculations and AIMD simulations. In addition, the constraint of Mg2+ on the first hydration shell is obviously stronger than that of Ca2+, while the constraint on the inner hydration shells fades slightly faster for Mg2+ than Ca2+. It is also found that the charge transfer from central cation to water molecules is affected only by the first hydration shell for Mg2+, whereas by the first and second hydration shells for Ca2+. Based on hydration characteristics, approximatively saturated ion hydration shells for the hydration of Mg2+ and Ca2+ were proposed.