  The formation of crystals has proven to be one of the most challenging phase
transformations to quantitatively model - let alone to actually understand - be
it by means of the latest experimental technique or the full arsenal of
enhanced sampling approaches at our disposal. One of the most crucial
quantities involved with the crystallization process is the nucleation rate, a
single, elusive number that is supposed to quantify the average probability for
a nucleus of critical size to occur within a certain volume and time span. A
substantial amount of effort has been devoted to attempt a connection between
the crystal nucleation rates computed by means of atomistic simulations and
their experimentally measured counterparts. Sadly, this endeavour almost
invariably fails to some extent, with the venerable classical nucleation theory
typically blamed as the main culprit. Here, we review some of the recent
advances in the field, focusing on a number of perhaps more subtle details that
are sometimes overlooked when computing nucleation rates. We believe it is
important for the community to be aware of the full impact of aspects such as
finite size effects and slow dynamics, that often introduce inconspicuous and
yet non-negligible sources of uncertainty into our simulations. In fact, it is
key to obtain robust and reproducible trends to be leveraged so as to shed new
light on the kinetics of a process, that of crystal nucleation, which is
involved into countless practical applications, from the formulation of
pharmaceutical drugs to the manufacturing of nano-electronic devices.
