Can a computer crystallize a liquid? Molecular simulation of continuous trajectories from liquid to crystalline n-hexane

A new algorithm is described for the simulation of the organization processes that lead from an isotropic liquid to a crystal structure. The algorithm is based on a hybrid, pseudo-Monte Carlo technique that lets a molecular assembly evolve under Metropolis conditions subject to forcing the decrease of asymmetry parameters that quantify the deviation from intermolecular alignment. This procedure was applied to the simulation of the liquid-solid transformation in n-hexane. Starting from liquid n-hexane with a significant population of gauche molecular conformations, a crystalline molecular assembly with parallel all-trans aliphatic chains is generated. This structure is very similar to the real crystal structure, except for a small difference in molecular orientation within layers. The process apparently involves an activation energy of a few kJ mol -1 and a very minor volume activation that could be associated with density fluctuations due to the need for extra space as long molecules undergo a large conformational rearrangement. Although some detailed quantitative aspects of the simulation may be open to discussion, this is one of the very few examples of individuation of a continuous all-atom trajectory linking the liquid and the solid state of a large polyatomic molecule. The results confirm the viability of the symmetry bias for the generation of new crystal structures, and provide valuable working hypotheses on the mechanism of crystallization.