The thermodynamics of water incorporation into calcium carbonate to form hydrates has been computed quantum mechanically using density functional theory (DFT). The structure of both the hydrated and the anhydrous phases are accurately reproduced by pure-DFT, hybrid Hartree–Fock/DFT, and DFT-D2 (long-range empirical correction). However, all of the aforementioned schemes fail to correctly reproduce the experimental energetics for the hydration process. In particular, functionals that provide reliable values for the anhydrous and low water content phases (calcite, aragonite, monohydrocalcite) fail to predict the energetics for the highly hydrated phase (ikaite) and vice versa, such that a comprehensive reliable study cannot be performed with a single method. Given that the available C6 parameters for the dispersive contributions in augmented DFT schemes typically are derived for atoms in molecular environments, we have refitted this parameter specifically for carbonates based on the relative enthalpy of ara...