Nucleation Statistics from Experiments as a Benchmark for Theory and Simulations

The fate of a metastable substance is governed by rare nucleation events, whose full microscopic details still remain elusive despite over 150 years of study. Experimental nucleation rates often differ by many orders of magnitude from theory and simulations, limiting predictive power. In this Perspective, after a general overview on nucleation, we focus on crystallization in three representative systems: metastable water, colloidal suspensions, and Lennard-Jones liquids. The latter, which are well realized by rare-gas liquids, provide a touchstone for nucleation theory. Recent femtosecond X-ray diffraction experiments on supercooled argon and krypton deliver accurate crystal nucleation statistics and direct insight into structural defects such as stacking faults. These advances establish rare-gas liquids as uniquely well-controlled systems bridging experiment, simulation, and theory, and pave the way toward a more complete microscopic understanding of nucleation.