  The Classical Nucleation Theory (CNT) has played a key role in crystal
nucleation studies since the 19th century and has significantly advanced the
understanding of nucleation. However, certain key assumptions of CNT, such as a
compact and spherical nucleating cluster and the concept of individual
diffusive jumps are questionable. The results of molecular dynamics (MD)
studies of crystal nucleation in a Al20Ni60Zr20 metallic liquid demonstrate
that the nucleating cluster is neither spherical nor compact. The seeding
method was employed to determine the critical cluster size and nucleation
parameters from CNT, which were then compared to those derived from the Mean
First Passage Time (MFPT) method. While the CNT-based nucleation rate aligns
well with experimental data from similar metallic liquids, the MFPT rate
differs significantly. Further, contrary to the assumption of individual jumps
for atoms to join the nucleating cluster, a cooperative mechanism of attachment
or detachment is observed. This is accompanied by synchronized changes in the
local potential energy. Similar cooperative motion also appeared in a
non-classical nucleation process, particularly during the coalescence of
nuclei.
