Homogeneous nucleation and the Ostwald step rule

We compare the pathways for homogeneous nucleation in a number of different systems. In most cases, the simulations show that the nucleation pathways are markedly different from what is assumed in classical nucleation theory. We find that homogeneous nucleation exhibits, at the microscopic level, features that were formulated over a century ago by Ostwald to account for the nucleation of metastable bulk phases. We observe that the structure of the precritical nuclei may differ qualitatively from that of (post)critical nuclei. Surprisingly, in the interface of the larger nuclei, traces of the structure of the smaller nuclei survive.