  Dislocation nucleation is essential to our understanding of plastic
deformation, ductility and mechanical strength of crystalline materials.
Molecular dynamics simulation has played an important role in uncovering the
fundamental mechanisms of dislocation nucleation, but its limited time scale
remains a significant challenge for studying nucleation at experimentally
relevant conditions. Here we show that dislocation nucleation rates can be
accurately predicted over a wide range of conditions by determining the
activation free energy from umbrella sampling. Our data reveal very large
activation entropies, which contribute a multiplicative factor of many orders
of magnitude to the nucleation rate. The activation entropy at constant strain
is caused by thermal expansion, with negligible contribution from the
vibrational entropy. The activation entropy at constant stress is significant
larger than that at constant strain, as a result of thermal softening. The
large activation entropies are caused by anharmonic effects, showing the
limitations of the harmonic approximation widely used in solids. Similar
behaviors are expected to occur in other nucleation processes in solids.
