A Molecular Study of CaCO$_3$ cluster configurations

Equilibrium relationships involving solids are based on bulk thermodynamic properties that concern ideal crystals of infinite size. However, real processes towards equilibrium imply development of finite molecular-scale entities. The configuration of these early-stage clusters and the estimation of their excess energies with respect to the ideal crystal are keys to understanding the macroscopic behaviour of a given system. As nucleation events are difficult to study experimentally, both because they occur spontaneously and because the nucleus size is very small, atomistic simulations are a suitable tool for understanding the early stages of crystallisation. Here, starting from the ideal atomic positions in calcite and aragonite, the relaxation in vacuum of finite clusters of CaCO$_3$ is explored. Nucleation and growth of calcium carbonate phases constitute a very important subject of research in a wide variety of fields. A complete study of CaCO$_3$ should include many different aspects: size and shape of the critical nuclei under diverse conditions, possibility of nucleation from precursor phases, nucleus energy and nucleus surface energy, relationship nucleus-substrate in heterogeneous nucleation...We present a preliminary study of nucleation of calcium carbonate where nuclei are considered to be isolated from any previous phase or substrate. Even when this situation does in no way represent realistic conditions, it can be a helpful first approach to more complex studies.