In many cases, a process yields crystals of more than one crystalline structure (or polymorph) at the same time, making the control of polymorphism extremely challenging. Several mechanisms have been proposed to account for this phenomenon, known as concomitant polymorphism. It has been attributed either to competing processes of homogeneous nucleation of different polymorphs or to solvent-mediated conversion of one polymorph into another or, more recently, to the heterogeneous nucleation (or cross-nucleation) of one polymorph on another. In recent work [1-4], we used Hybrid Monte Carlo (HMC) simulations as well as molecular dynamics simulations to study the early stages of crystallization in a supercooled liquid of spherical particles. We carry out two different simulation methods corresponding to the two mechanistic steps of crystal nucleation and growth. To simulate the formation of a nucleus of a critical size, we combine HMC simulations together with a method suited to study rare events such as the umbrella sampling technique. The growth step is simply simulated using unconstrained molecular dynamics simulations. We observed the onset of concomitant polymorphism and demonstrated that this phenomenon resulted from the cross-nucleation of a metastable polymorph on the stable polymorph. Our simulations provide detailed insights into the molecular mechanism underlying concomitant polymorphism and cross-nucleation. We discuss the significance of this phenomenon and of its effects on both crystal nucleation and growth.

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

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

[3] C.Desgranges and J. Delhommelle, J. Phys. Chem. B 111, 1465 (2007).

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