Accurately describing material properties during processes that involve bond breaking, for example fracture or a chemical reaction, requires an atomistic description of the material, and a quantum-mechanical description of the electrons that mediate bonding. Density functional theory has made it feasible to carry out such simulations reliably from first principles, but is limited in practice to a few hundred atoms. However, in many interesting systems the localized bond breaking process is coupled to a very large number of atoms. In fracture this coupling is mechanical, to the applied strain field, while for reactions in water the bond breaking is coupled electrostatically to the transient hydrogen bond network of the solvent molecules. I discuss the issues that occur in trying to treat such coupled systems, and describe the buffered force-mixing method we have developed to overcome these problems. I present results on the first application of the method, to fracture in silicon, a model brittle solid, where it predicts a low speed instability that can be linked to experimentally observed features of the crack surface. I also show how the method has been extended to describe reactions in liquids, where it allows solvent molecules to affect the reaction and diffuse into and out of the quantum-mechanical region. I present results for the structure of liquid water and a proton transfer reaction in water that shows good agreement with fully quantum-mechanical calculations, in contrast with conventional coupling methods.