When simulating an ion in water, one would assume that a smaller ion is always easier to insert than a large one. This is the case for water at the density . However, when we stretch water so that its density is lowered, we observe an opening of the water structure; the space available to an ion grows. Very small ions tend to counteract this opening of the structure because they try to reorient the surrounding water molecules with the water molecule's charge concentrations towards the charge of the ion and to pull these reoriented molecules inward, tending to collapse the structure. A larger ion (the appropriate size depending on the density of the system) can however stabilize the open structure. This is due to the fact that a larger ion can ``fill'' the hole much easier without pulling water molecules towards itself. The hydration shell of the larger ion now has some similarity with the hydration shell of a noble gas. The water molecules (depending on the density) do not reorient towards the ion in the same way for stretched water as they do for regular water with a density of . Because of this, the water molecules can still make a small contribution to the hydrogen bond network.
System settings :
Start with the NVT ensemble; fix the temperature at 298 K and the density at . Insert an ion and now slowly decrease the density in increments of .