The energies obtained from orthorhombic systems in group C are higher than those from monoclinic structures in group D, suggesting that orthorhombic systems represent transition states or nonviable structures. Indeed, the density of these structures is higher (∼2.70 g/cm 3 ) than that of experimental vaterite data (∼2.66 g/cm 3 ). The P 2 1 /c SG found in group C represents a slightly tilted structure compared to orthorhombic cells. The CO 3