Molecular-scale interactions present at mineral-water interfaces and in clay interlayer galleries control numerous environmental processes, including chemical interactions in soils and transport of nutrients and pollutants through them.[1-4] Understanding these processes requires accurate knowledge of the structure, energetics, and dynamics of the interaction among the mineral substrate, ions, and water molecules.[5, 6] Challenges to this objective include experimental difficulties in probing these interfaces and interlayers at the molecular scale; fully characterizing the mineral substrate; and identifying how the mineral surface, ions, and water molecules each contribute to the overall structure, energetics, and dynamics of these systems.[6] Linked computational molecular dynamics (MD) simulations and experimental nuclear magnetic resonance (NMR) studies are particularly effective in addressing these issues.[7-9] Here we focus on MD studies of Na- and Ca-smectite (hectorite) interlayer galleries to provide a molecular-scale picture of the structure and dynamics of their hydration[9, 10] and to complement our earlier NMR investigations of these systems.[7-9] Classical MD simulations were undertaken in the NPT and NVT ensembles to determine the structural and energetic changes with increasing hydration with focus on the single- and double-layer hydrates. The results show substantial changes in the hydration of the interlayer cations, the orientations of the water molecules, the hydrogen bond network involving the water molecules and basal oxygen atoms, and the resulting potential energies as the interlayer gallery expands. [1] Scheidegger et al. (1996) Soil Science 161 813-831. [2] Stumm (1997) Colloids and Surfaces A-Physicochemical and Engineering Aspects 120 143-166. [3] O'Day (1999) Reviews of Geophysics 37 249-274. [4] Koretsky (2000) Journal of Hydrology 230 127-171. [5] Wang et al. (2001) Chemistry of Materials 13 145-150. [6] Wang et al. (2006) Geochimica et Cosmochimica Acta 70 562-582. [7] Bowers et al. (2008) Journal of Physical Chemistry C 112 6430-6438. [8] Bowers et al. (2011) Journal of Physical Chemistry C 115 23395-23407. [9] Bowers et al. (2012), unpublished. [10] Morrow et al. (2012) Journal of Physical Chemistry C, submitted.