  The solvation structures and ion dynamics of CaCl$_2$ aqueous electrolytes
have been investigated using ab initio molecular dynamics simulations and
molecular dynamics simulations with deep learning potentials. We found multiple
solvation structures around the Ca$^{2+}$ ion, including fully hydrated single
Ca$^{2+}$ ion, Ca-Cl contact ion pair, and Ca-2Cl bridged ion pair, could
coexist. The ion-pairing condition plays an important role in the translational
and orientational distribution of water molecules in the solvation shell. And
the local ordering introduced by the Ca$^{2+}$ ion can extend to the second
solvation shell. Furthermore, we found the lifetime of water molecules in the
solvation shell is sensitive to the ion-pairing conditions. The
self-diffusivities of ions and water molecules, as calculated in molecular
dynamics simulations with deep learning potentials, are in good agreement with
experimental measurements. Finally, we elucidate the transition of Ca$^{2+}$
ion dynamics between different regimes by analyzing angle probability
distribution histograms and van Hove correlation function.
