The free energy difference between the bulk black and yellow phases provides the driving force for nucleation, while the interfacial energy impedes the formation of nuclei. We verify that the classical force field can accurately describe the interfacial energy between different structures (supplementary material, Table ). For the one-step C-C transition mechanism, σ corresponds to the interfacial energy of the black/yellow (B/Y) interface (Fig. , red box), calculated to be 1.4