Car–Parrinello molecular dynamics simulation of the hydrated calcium ion

First principles molecular dynamics has been used to investigate the structural, vibrational, and energetic properties of $[Ca(H2O)n]2+$ clusters with $n=1–9,$ and the hydration shell of a calcium ion in a periodically repeated box with 54 water molecules. We find that, while stable highly symmetric Ca–water clusters can be formed with up to eight water molecules, the $n=9$ cluster dissociates into the last stable $[Ca(H2O8]2+$ complex. In solution the first hydration shell around the $Ca2+$ ion contains six water molecules in an octahedral arrangement. The electronic structure of nearest neighbor hydration shell water molecules has been examined with a localized orbital analysis. The average dipole moments of hydration water molecules was found to be increased by about 0.4 Debye relative to that of pure water.