  Ice nucleation is a process of great relevance in physics, chemistry,
technology and environmental sciences, much theoretical and experimental
efforts have been devoted to its understanding, but still it remains a topic of
intense research. We shed light on this phenomenon by performing atomistic
based simulations. Using metadynamics and a carefully designed set of
collective variables, reversible transitions between water and ice are able to
be simulated. We find that water freezes into a stacking disordered structure
with the all-atom TIP4P/Ice model, and the features of the critical nucleus of
nucleation at the microscopic level are revealed. Our results are in agreement
with recent experimental and other theoretical works and confirm that
nucleation is preceded by a large increase in tetrahedrally coordinated water
molecules.
