eV/nm 2 (supplementary material, computational methods, force field validation, Table , and Fig. ). The red solid line in Fig. shows the variation of ∆G with R, predicting that if the B↔Y phase transition occurs via the one-step C-C transition, the critical radius of the nucleus is 5 nm, and the nucleation barrier reaches 150 eV! For the multi-step C-C phase transition mechanism , σ can be reduced to 0.8 eV/nm 2 (Fig. , yellow box) , but the corresponding nucleation barrier still reaches 25 eV (yellow line in Fig. ). Such high nucleation barriers are far above the maximum barrier allowed by thermally activated nucleation processes (∼80 k B T ) , making the possibility of phase transition via the C-C mechanism extremely low.