Formation of Alkali Metal/Alkaline Earth Cation Water Clusters, M(H2O)1-6, M = Li+, Na+, K+, Mg2+, and Ca2+: An Effective Fragment Potential (EFP) Case Study

Abstract

To assess the ability of the effective fragment potential (EFP) method to describe the hydration of simple cations, calculations have been carried out on alkali metal (Li+, Na+, and K+)/alkaline earth (Mg2+ and Ca2+) cation water clusters containing up to six water molecules. The restricted Hartree−Fock (RHF) and second-order Moller−Plesset (MP2) perturbation methods have also been employed in the study. It is shown that the EFP method is capable of accurately reproducing RHF differential standard enthalpies of hydration for the alkali metal and calcium cation water systems. The EFP method also reproduces experimental differential and total standard enthalpies of hydration for the alkali metal cation water complexes. Good agreement is also found among the EFP and ab initio levels of theory for the enthalpies of the calcium cation water clusters. Possible reasons for discrepancies between the EFP results and those obtained at the ab initio levels for the structures of the Na+(H2O)1-6 and Mg2+(H2O)1-6 clust...