It is important that CVs approximate the reaction coordinate associated with the nucleation process and therefore distinguish important states along the reaction path, such as reactants, products and, ideally, configurations belonging to the transition state ensembles. Moreover, to be used in biased enhanced sampling simulations, CVs should be continuous and differentiable functions of the atomic coordinates. It was demonstrated by Peters et al. that to study single-step nucleation mechanisms and to determine rates, CVs should characterise the size of the largest crystalline cluster in solution; indeed, the application of CVs to describe phase separation can be tested to ensure they are good ones to determine mechanisms and rates and to compare to reaction coordinates adopted in established nucleation theories. . In two-step processes, a two-dimensional CV space representing the extent of the largest cluster and of the largest ordered domain in the nucleating phase have also emerged as good descriptors of the reaction coordinate , which also lend themselves to a theoretical description of two-step nucleation .