  Here we propose an NPT metadynamics simulation scheme for pressure-induced
structural phase transitions, using coordination number and volume as
collective variables, and apply it to the reconstructive structural
transformation $B1$-$B2$ in NaCl. By studying systems with size up to 64 000
atoms we reach a regime beyond collective mechanism and observe transformations
proceeding via nucleation and growth. We also reveal the crossover of the
transition mechanism from Buerger-like for smaller systems to
Watanabe-Tol\'{e}dano for larger ones. The scheme is likely to be applicable to
a broader class of pressure-induced structural transitions, allowing study of
complex nucleation effects and bringing simulations closer to realistic
conditions.
