Except for the energy barrier as discussed above, another vital factor is that the stable Zn 2+ -F -outer-sphere complexes exist in the from 2SSIPs rather than SSIPs at the room temperature (300 K), as observed in the energy profile of Fig. where the energy trough locates at the range of 5.1-5.7 Å of Zn 2+ -F -distance. This is markedly against common sense in which the SSIPs are favorable due to the strong electrostatic attraction between M 2+ and F -. When Zn 2+ is replaced with Mg 2+ , having same charge and ion radius as Zn 2+ , the dissociation energy profile shows that Mg 2+ -F -SSIPs are stable outer-sphere complexes (energy trough locates at the range of 3.8-4.5 Å of Mg 2+ -F - distance.) . To further confirm this anomaly, both clusters, Zn 2+ -F -SSIP and 2SSIP with explicit water molecules (40 H2O), extracted from dynamic trajectory are optimized by static DFT calculations. The energy with zero point correction of 2SSIP is lower than SSIP by ~2.31 kcal•mol -1 , supporting the observation of Fig. . The result suggests that the interaction of Zn 2+ and F -is beyond simple electrostatic attraction effect and different from that of Mg 2+ and F -. Analogous difference is also observed in their chloride congener. The results of Duboué-Dijon et al. showed that the neutron scattering of aqueous MgCl2 solutions could be well reproduced by electronic continuum correction scaled-charge force field but a specific interaction term between the ions must be introduced when applying such a force field to describe ZnCl2 solutions .