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Modeling semi-brittle fracture is a multiscale task. A first approach could bemade in two dimensions by coupling discrete dislocation dynamics (DD) andmolecular dynamics (MD). The problem is that dislocations are not static in MDwhereas, in DD, equilibrium elastic solutions and superposition are usually used.We have recently proposed new equations of motion for MD which give fullcontrol on dislocation mobility. An order parameter is used to measure the localshear and use it as a holonomic constraint, in the same spirit as the constraintsused in the dynamics of molecules with rigid bonds. In this paper, we show twoapplications : constraining dislocation emission from a crack tip (intrinsicductility) and the calculation of crack tip shielding from a pinned dislocation. Weshow that DD and MD are in excellent agreement, which opens the possibility ofstudying complex dislocation – crack tip arrangements, multiscale.