Wei Cai Stanford University Seunghwa Ryu1, Keonwook Kang2, and Wei Cai1 1Department of Mechanical Engineering, Stanford University, California 94305, USA 2Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 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 timescale remains a significant challenge for studying nucleation at experimentally relevant conditions. Classical nucleation theories have been used to predict the nucleation rate from activation energy calculations, but the magnitude of the activation entropy has remained unknown. We show that dislocation nucleation rates can be accurately predicted over a wide range of conditions by Becker-Döring theory of nucleation with the activation free energy calculated 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 significantly 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 for rate estimation in solids.