Taken all together, our simulations identify the initial formation of the SV as the bottleneck step of the a-and b-pathways, both at 2.9 and 6.8 GPa, requiring a bias of 870-1160 kJ/mol. Such bias values, even though they suggest that these transformation pathways are unlikely to be observed at room temperature, should be considered as (possibly severe) overestimates of the equilibrium free-energy barriers for the methodological reasons explained above.