All atoms in the framework were held fixed at their crystallographic positions. The number of unit cells used was different for each structure to ensure that the perpendicular lengths of the supercell were greater than twice the cutoff used. The cutoff for Lennard-Jones and charge-charge short-range interactions was R c = 12.8 Å and the Ewald sum technique was applied to compute the long-range electrostatic interactions on a 0.1 Å grid with a relative precision of 10 -6 . The Lennard-Jones potential was shifted to zero at the cutoff.