  Bismuth segregated to the grain boundary in Cu is known to promote brittle
fracture of this material. Recently, Schweinfest et al. reported
first-principles quantum mechanical calculations on the electronic and
structural properties of a Cu grain boundary with and without segregated Bi and
argue that the grain boundary weakening induced by Bi is a simple atomic size
effect. But their conclusion is invalid for both Bi and Pb because it fails to
distinguish the chemical and mechanical (atomic size) contributions, as
obtained with our recently developed first-principles based phenomenological
theory.
