Polymorphism is the ability of a molecule to crystallize in different structures or polymorphs. Polymorphs of the same molecule may exhibit very different physical properties such as e.g. the solubility and hence the bioavailability of drug molecules. This phenomenon is thus of great technological importance in the making of drugs or pigments as well as in the food industry. The control of polymorphism remains a long-standing and challenging issue and a complete understanding of this phenomenon still eludes us, even for simple systems of spherical particles. Because critical nuclei are small, short-lived and thus difficult to observe directly in experiments (with the notable exception of colloidal systems, which may be observed using confocal microscopy), molecular simulation is a natural tool to investigate this phenomenon.

In this talk, we present simulation results obtained on a variety of model systems (Lennard-Jones [1], Yukawa [2], exp-6 [3] as well as inverse-power-law [4]potentials). We carry out two different types of simulation corresponding to the two mechanistic steps of the crystallization process, i.e. the nucleation and the growth steps. To simulate the activated process of crystal nucleation, we combine Hybrid Monte Carlo simulations together with an umbrella sampling potential while for the growth step, we simply carry out unconstrained molecular dynamics simulations. We show and rationalize how, by modifying the conditions of crystallization, we are able to favor the formation of large crystallites either of the stable polymorph or of the metastable polymorph.

[1] C.Desgranges and J. Delhommelle, Phys. Rev. Lett. 98, 235502 (2007).

[2] C.Desgranges and J. Delhommelle, J. Am. Chem. Soc. 128, 15104 (2006).

[3] C.Desgranges and J. Delhommelle, Phys. Rev. B 77, 054201 (2008).

[4] C.Desgranges and J. Delhommelle, J. Phys. Chem. B 111, 12257 (2007).