In disordered packings, local particle displacements deviate from the global affine strain. Non-affine displacement D² quantifies these fluctuations — largest near soft modes and rearrangement events.
Falk & Langer (1998): non-affine displacement D²_min measures local deviation from best-fit affine deformation. Zones of high D² correlate with shear transformation zones (STZ) — structural rearrangements that mediate plastic flow in glasses.