A 20 ps portion of the Cs-montmorillonite, 1/3 water monolayer MD simulation animation (65–85 ps). Interlayer species are represented as described in Plate 1. The two highlighted Cs+ (red) share the two highlighted water molecules (lighter blue). The molecules making up this two Cs+–two water complex are so strongly bound that they are observed to rotate as a unit during this animation. Interlayer water molecules do not enter six-oxygen cavities in this portion of the simulation. See Additional file 2 to access the animation.