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Calcium oxalate is found in nature in different crystalline forms with variable levels of hydration1-4,namely calcium oxalate monohydrate(whewellite,CaC2O4·H2O,COM),calcium oxalate dihydrate(weddellite,CaC2O4·(2+x)H2O,x≤0.5,COD)and more rarely as calcium oxalate trihydrate(caoxite CaC2O4·3H2O,COT).Recently an amorphous phase of calcium oxalate has also been observed experimentally5-6,and it has been suggested that it can be a precursor to the formation of the crystalline phases.However,at present the actual molecular mechanisms controlling the formation of calcium oxalate in aqueous solution are still unknown.The main aim of the present research is to understand the nucleation and the growth mechanisms of calcium oxalate in aqueous solution through classical molecular dynamics simulation.The first step was to develop an accurate force field that is capable of capturing the thermodynamic properties of the ions in solution and of the crystalline phases.Subsequently,we investigated the aggregation of ions in solution and used metadynamics7 simulations to calculate the pairing free energy for Ca2+ and C2O4 2-as a function of the distance between the ions and the calcium coordination number by water(as presented in Figure 1).The stability of small calcium oxalate clusters has also been investigated using metadynamics as well as other computational techniques.In order to validate the results from the classical force field,the Ca2+-C2O4 2-pairing free energy has also been investigated by density functional theory(DFT)based molecular dynamics.