The data given in the paper were obtained using CASTEP based on the density functional theory (DFT) applying a basis set of plane waves and PBE exchange-correlation functional. Van der Waals interactions were considered by the Grimme-D2 semi-empirical correction. The data include the optimized geometry and electronic properties of the equilibrium state of the non-hydrated cis- and trans-vacant variety of a Na-montmorillonite (MMT) and its state after the adsorption of water molecules. The data on hydration shells formed by the Na+ cation on the basal surface of MMT are also presented. The data are presented on the behavior of crystalline hydroxyl groups and water molecules during their adsorption. Data files of the optimized crystal structures and electronic properties can be read by the public text editors.
