ABSTRACT
Brittle fracture is a nice example of intrinsically multi-level problems in material science. On the large scale one has to correctly take into account the driving force of the crack that depends on the elastic properties of the whole system. But simultaneously, it is also necessary to accurately describe the bond disruption processes that take place at the atomic scale. In strongly covalent materials the quantum precision in the vicinity of the crack tip is particularly crucial. In the case of diamond silicon the use of purely classical potentials leads to qualitatively wrong descriptions of the crack dynamics showing for example plastic deformations, high fracture energy release rates, unphysical crack tip structures, large surface roughness and even crack blunting and wrong crack propagation direction. On the other hand full quantum calculations remain practically untractable on the several thousands of atoms needed to integrate the slowly decaying crack elastic field.
