Density functional theory and interatomic potential study of structural, mechanical and surface properties of calcium oxalate materials

The structural and elastic properties of anhydrous calcium oxalate [COA, Ca(C2O4)] and calcium oxalate monohydrate [COM, Ca(C2O4)·H2O] have been studied by first-principles and interatomic potential calculations. Density functional theory calculations of the structures of COA and COM using a semi-empirical addition of dispersive forces to the Perdew–Burke–Ernzerhof (PBE) functional (PBE-D) are in substantially better agreement with experiment than conventional PBE calculations. The single-crystal elastic stiffness constants Cij of COA and COM have been computed at the PBE-D level from the polynomial fit of the total energy curve as a function of the deformation. We have consequently derived an interatomic potential (IP) for calcium oxalate that accurately reproduces the structural and elastic properties (bulk modulus, shear modulus, Young's modulus, bulk modulus-shear modulus ratio, Poisson's ratio, and elastic anisotropy ratio) of COA and COM as predicted by PBE-D. The IP model has been applied to compute the surface energies of COM and determine its equilibrium morphology by applying the Gibbs–Wulff theorem. The computed morphologies of the COM crystal agree with experimentally found morphologies.