Insight into the nucleation of urea crystals from the melt

Obtaining molecular-level information regarding nucleation is an essential step towards a thorough comprehension of crystallization processes. In this work we investigate the nucleation of urea at the atomic scale using enhanced sampling Molecular Dynamics simulations. We show that by employing a set of suitably defined collective variables in a Well Tempered Metadynamics scheme it is possible to reversibly drive the system across the solid-liquid phase transition and to recover the associated free energy surface. Our study reveals the presence of an undiscovered metastable ordered structure competing with the experimental one during crystal nucleation, suggesting a non-classical mechanism for this process.