It is highly unlikely that any single-step, homogeneous crystal nucleation occurs according to the above mechanism. The capillary approximation is particularly problematic for crystal nucleation because the smallest clusters are unlikely to display an interface structure with surface tension that matches highly faceted bulk crystals with infinitely large planar surfaces. Furthermore, it may not be the case that the density and structure in the emerging crystal are homogeneous throughout. The CNT framework also fails to account for the role that growth units beyond monomers might play in the formation of crystal nuclei. Some of the effects described above can be accounted for in frameworks which we label as extended classical nucleation. For example, models that depart from CNT include a surface tension term that is dependent on the cluster size. Other theoretical and simulation studies have identified that the structure, particularly the density, of the smallest nuclei differs from the bulk and stated that this should be accounted for in extensions of CNT. Formulations based on classical density functional theory provide corrections to the CNT nucleation rate derived from the excess system free energy that accounts for varying cluster density. Given the description of two-step nucleation above, it may be perceived that this type of phase separation mechanism is inconsistent with classical nucleation. However, theoretical studies have demonstrated that two-step nucleation can be described using classical concepts adopting a composite cluster model . Changes to the relative supersaturation of the system with respect to a dense liquid and crystal phase lead to changes in the pathways to crystals from solutions : a common observation in experimental studies.