turn, step stiffness plays a central role in our understanding of how steps respond to fluctuations and driving forces. It is one of the three parameters of the step-continuum model,[@JW] which has proved a powerful way to describe step behavior on a coarse-grained level, without recourse to a myriad of microscopic energies and rates. As the inertial term, stiffness determines how a step responds to interactions with other steps, to atomistic mass-transport processes, and to external driving forces. Accordingly, a thorough understanding of stiffness and its consequences is crucial.

The step stiffness $\tilde{\beta}$ weights deviations from straightness in the step Hamiltonian. Thus, it varies inversely with the step diffusivity, which measures the degree of wandering of a step perpendicular to its mean direction. This diffusivity can be readily written down in terms of the energies $\varepsilon_k$ of kinks along steps with a mean orientation along close-packed directions ($\langle 110 \rangle$ for an fcc (001) surface): in this case, all kinks are thermally excited. Conversely, experimental measurements of the low-temperature diffusivity (via the scale factor of the spatial correlation function) can be used to deduce the kink energy. A more subtle question is how this stiffness depends on the azimuthal misorientation 