Hydration and, in particular, coordination number (CN) of a metal ion, is of paramount importance
as it defines many of its (bio)physico-chemical properties. It is not only essential for understanding
its behavior in aqueous solutions but also determines the metal-ion reference state and its
binding energy to (bio)molecules. For the following divalent metal cations — Ca2+, Cd2+, Cu2+,
Fe2+, Hg2+, Mg2+, Ni2+, Pb2+, and Zn2+ — we compare here two approaches for predicting hydration
numbers: (1) a mixed explicit/continuum DFT-D3//COSMO-RS solvation model and (2)
DFT-based ab initio molecular dynamics (AIMD). The former approach is employed to calculate
Gibbs free energy change for the sequential hydration reactions, starting from [M(H2O)2]2+ aqua
complexes up to [M(H2O)9]2+ allowing explicit water molecules to bind in the first or second coordination
sphere and determining the most stable [M(H2O)n]2+ structure. In the latter approach,
the hydration number is obtained by integrating the ion-water radial distribution function. With
a couple of exceptions, the metal ion hydration numbers predicted by the two approaches are in
mutual agreement, as well as in agreement with the experimental data.