  Recent advances in classical density functional theory are combined with
stochastic process theory and rare event techniques to formulate a theoretical
description of nucleation, including crystallization, that can predict
nonclassical nucleation pathways based on no input other than the interaction
potential of the particles making up the system. The theory is formulated
directly in terms of the density field, thus forgoing the need to define
collective variables. It is illustrated by application to diffusion-limited
nucleation of macromolecules in solution for both liquid-liquid separation and
crystallization. Both involve nonclassical pathways with crystallization, in
particular, proceeding by a two-step mechanism consisting of the formation of a
dense-solution droplet followed by ordering originating at the core of the
droplet. Furthermore, during the ordering, the free-energy surface shows
shallow minima associated with the freezing of liquid into solid shells, which
may shed light on the widely observed metastability of nanoscale clusters.
