Molecular Simulation of Crystal Nucleation and Growth in Solution

We report a computer simulation study of the nucleation free energy and the structural evolution of sub-cooled, Lennard-Jones particles crystallized from solution. Umbrella sampling was employed to induce the initial formation of the critical nucleus, and trials with different Lennard-Jones cross interaction parameters  were carried out to assess the effects on the nucleation free energy. The systems with larger  were found to have much less phase separation and a larger nucleation free energy than the systems with smaller . Coincidently, the systems with the smaller  were completely separated into two distinct liquid phases. Also, the pre-critical nuclei were found to be mainly body-centered cubic (bcc) ordered. However, as the nucleus began to grow, face-centered cubic (fcc) structure was observed at the core, while bcc structure was found on the liquid-crystal interface. As the crystal reached larger sizes, fcc structure dominated. Our results are the first steps to developing new methods to control both the polymorph and rate of nucleation.