Swelling clays are known to exhibit crystalline swelling at the layer scale (∼ nm): X-Ray Diffraction (XRD) identifies discrete jumps in inter-layer spacing upon change in relative humidity (RH) corresponding to the thickness of a water layer. Continuum theories are not adapted to describe systems at the scale of a few water molecules, and much effort has been dedicated to understand and predict crystalline swelling from molecular simulations. Grand canonical (GC) molecular simulations combined with thermodynamic stability analysis are one of the most appropriate techniques to predict swelling as a function of RH from atomistic simulations. Yet, the usual GC Monte Carlo is quite inefficient, and improved algorithms are needed to reach reasonable accuracy. In this work, we report results of configurational bias GC molecular simulations of Na-montmorillonite, one of the most studied swelling clays. This data set covers a wide range of relative fugacities above and below vapor saturation and a wide range of spacings, encompassing all hydration states. This makes it possible to derive a complete hydration diagram as a function of external pressure and relative fugacity. Such a diagram extends literature results to undersaturated conditions and can be used to anticipate the crystalline swelling upon any complex loading. Of particular interest is free swelling upon a change in RH, for which our predictions compare well with experimental XRD results available in the literature. The constitutive behavior of the confined water has been little investigated so far, and existing studies were limited to the undrained stiffness (at constant water content). Since the Gibbs–Duhem equation does not hold for confined water, proper characterization requires three independent moduli, in contrast with bulk water that is characterized by the bulk modulus only. Here, we provide estimates of the three moduli. The constitutive behavior of confined water significantly differs from that of bulk water, but the bulk behavior is recovered in the capillary domain at a spacing larger than 19 Å.