The Becke scheme can also be conceptualized as a form of Eq. ( ) in which w A (r) is a smoothed version of a Heaviside step function, which switches rapidly between w A (r) = 0 and w A (r) = 1 at the boundaries of the Voronoi polyhedra. In practice, our implementation of Becke's procedure uses the "atomic size adjustments" that are described in Ref. , in which a set of empirical atomic radii are used to adjust the boundaries of the Voronoi cells away from the midpoints of the internuclear vectors. As discussed below, this adjustment is crucial for systems with substantial size mismatch between nearby atoms.