  The crystal nucleation from liquid in most cases is too rare to be accessed
within the limited timescales of the conventional molecular dynamics (MD)
simulation. Here, we developed a "persistent embryo" method to facilitate
crystal nucleation in MD simulations by preventing small crystal embryos from
melting using external spring forces. We applied this method to the pure Ni
case for a moderate undercooling where no nucleation can be observed in the
conventional MD simulation, and obtained nucleation rate in good agreement with
the experimental data. Moreover, the method is applied to simulate an even more
sluggish event: the nucleation of the B2 phase in a strong glass-forming Cu-Zr
alloy. The nucleation rate was found to be 8 orders of magnitude smaller than
Ni at the same undercooling, which well explains the good glass formability of
the alloy. Thus, our work opens a new avenue to study solidification under
realistic experimental conditions via atomistic computer simulation.
