(183d) Mechanisms and Free Energy Barriers for the Nucleation of Molecular Crystals From the Melt and From Solution

Understanding crystallization is crucial for many applications in the food, chemical and pharmaceutical industries. Despite its widespread use, however, the design of crystalline materials is far from an easy task. With the exception of a few simple systems, the molecular-level details of the nucleation process are still poorly understood. On the one hand, the stochastic nature of nucleation, and the fact that it usually involves only a few molecules/ions, make it very hard to probe experimentally. On the other hand, the time scales involved are beyond the reach of traditional molecular simulation methods.

In this work, we combine a novel method to characterize the degree of order in molecular crystals [1] with the string method in collective variables [2] to obtain minimum free energy pathways for the nucleation of several systems from the melt and from solution. We then use milestoning with Voronoi tesselations [3] to obtain free energy barriers for nucleation. We compare some of the results from this approach with those obtained using transition path sampling. Finally, we show how this method can be extended to study aggregation in systems where the aggregates only exhibit partial ordering. These results are a first step toward a molecular-level understanding of nucleation in molecular systems.

[1] E.E. Santiso and B.L. Trout, J. Chem. Phys. 134, 064109 (2011)

[2] L. Maragliano et al., J. Chem. Phys. 125, 024106 (2006)

[3] E. Vanden-Eijnden and M. Venturoli, J. Chem. Phys. 130 194101 (2009)