  Slow relaxation and plastic deformation in disordered materials such as
metallic glasses and supercooled liquids occur at dynamical heterogeneities, or
neighboring particles that rearrange in a correlated, cooperative manner.
Dynamical heterogeneities have historically been described by a four-point,
time-dependent density correlation function $\chi_4 (r, t)$. In this paper, we
posit that $\chi_4 (r, t)$ contains essentially the same information about
medium-range order as the Van Hove correlation function $G(r, t)$. In other
words, medium-range order is the origin of spatially correlated regions of
cooperative particle motion. The Van Hove function is the preferred tool for
describing dynamical heterogeneities than the four-point function, for which
the physical meaning is less transparent.
