- The structure of high-velocity screw dislocations in Cu was investigated using molecular dynamics. We observed that the dissociation width of the dislocation into its partials is velocity dependent and identified two main regimes at low and high velocities with respect to the shear wave velocity. In the first regime the drag coefficient of the dislocation is temperature dependent and the dissociation width decreases with the velocity. In the second regime the drag coefficient is an order of magnitude higher than in the first regime and is temperature independent. In this regime the trailing partial spreads from the slip plane onto the cross-slip plane and the dislocation core becomes nonplanar. Due to this nonplanar structure, the dislocation widens as the velocity increases and the partial separation becomes unstable at velocities above 0.67 times the shear wave velocity. We derive an elastic-continuum model for the gliding dislocation structure.