Onset of Amorphous Structure in CaCO3: Geometric and Electronic Structures of (CaCO3)n (n = 2–7) Clusters by Ab Initio Calculations

Electronic structures, vibrational analyses, stabilization energies and amorphicity were analyzed for the most stable configurations of the neutral gas phase clusters of calcium carbonate (CaCO3)n (n = 2–7). Minimum-energy structures were generated through simulated annealing using a mix of molecular dynamics/semiempirical optimization and then full optimization at the ab initio level (RHF/6-31G*). HF-level results are calibrated versus MP2/6-31G* results. Though many clusters are symmetric, the structural arrangements are not crystalline and resemble neither calcite nor aragonite. In general, greater symmetry means a lower stabilization energy. The BSSE- and zero point energy-corrected stabilization energies appear to plateau at about −113 kcal/mol when n = 7. Amorphous clusters start appearing when n = 4 and the lowest-energy minima for n ≥ 5 are of C1 symmetry. No solvent effect is necessary to induce amorphism.