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)center dot 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 C-ij 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.