  A comparative study of fracture in Al is carried out by using quantum
mechanical and empirical atomistic description of atomic interaction at crack
tip. The former is accomplished with the density functional theory (DFT) based
Quasicontinuum method (QCDFT) and the latter with the original Quasicontinuum
method (EAM-QC). Aside from quantitative differences, the two descriptions also
yield qualitatively distinctive fracture behavior. While EAM-QC predicts a
straight crack front and a micro-twinning at the crack tip, QCDFT finds a more
rounded crack profile and the absence of twinning. Although many dislocations
are emitted from the crack tip in EAM-QC, they all glide on a single slip
plane. In contrast, only two dislocations are nucleated under the maximum load
applied in QCDFT, and they glide on two adjacent slip planes. The electron
charge density develops sharp corners at the crack tip in EAM-QC, while it is
smoother in QCDFT. The physics underlying these differences is discussed.
