We describe and test a novel molecular dynamics method which combines quantum-mechanical embedding and classical force model optimization into a unified scheme free of the boundary region, and the transferability problems which these techniques, taken separately, involve. The scheme is based on the idea of augmenting a unique, simple parametrized force model by incorporating in it, at run time, the quantum-mechanical information necessary to ensure accurate trajectories. The scheme is tested on a number of silicon systems composed of up to approximately 200 000 atoms.

When a brittle material is loaded to the limit of its strength, it fails by the nucleation and propagation of a crack(1). The conditions for crack propagation are created by stress concentration in the region of the crack tip and depend on macroscopic parameters such as the geometry and dimensions of the specimen(2). The way the crack propagates, however, is entirely determined by atomic- scale phenomena, because brittle crack tips are atomically sharp and propagate by breaking the variously oriented interatomic bonds, one at a time, at each point of the moving crack front(1,3). The physical interplay of multiple length scales makes brittle fracture a complex 'multi-scale' phenomenon. Several intermediate scales may arise in more complex situations, for example in the presence of microdefects or grain boundaries. The occurrence of various instabilities in crack propagation at very high speeds is well known(1), and significant advances have been made recently in understanding their origin(4,5). Here we investigate low-speed propagation instabilities in silicon using quantum-mechanical hybrid, multi-scale modelling and single-crystal fracture experiments. Our simulations predict a crack- tip reconstruction that makes low-speed crack propagation unstable on the ( 111) cleavage plane, which is conventionally thought of as the most stable cleavage plane. We perform experiments in which this instability is observed at a range of low speeds, using an experimental technique designed for the investigation of fracture under low tensile loads. Further simulations explain why, conversely, at moderately high speeds crack propagation on the (110) cleavage plane becomes unstable and deflects onto ( 111) planes, as previously observed experimentally(6,7)

We propose a detailed description of the structural and electronic properties of neutral and charged TinO2n+m clusters (n=1–3 and m=0,1), through simulations based on the density functional theory in the local spin density approximation. In all the isomers studied, strongly bound titanyl groups are found. The order of stability of the low-energy stoichiometric clusters may change considerably from that found by the approaches based on classical electrostatics. The most stable isomers of the oxygen-rich neutral clusters show characteristic peroxide groups. All these facts stress the importance of the covalent contribution to the cohesion of the clusters. Large atomic relaxations, accompanying the change from a closed-shell to an open-shell electronic configuration when an electron is added or removed, can often induce reversals of stability among the isomers. A careful discussion of the computed electron affinities and excitation energies as a function of the size and the atomic conformation of the clusters is performed, in relation to recent experimental data.

The mechanism of plastic flow in amorphous solids involves nucleation-controlled shear transformations, triggered under stress from fertile sites. However, the origin of these sites is still a matter of debate. In this paper, we show that the connection between local plastic activity and coordination defects in amorphous systems depends on the nature of the interatomic interactions. In particular, the directionality of the bonds, as quantified by the three-body term in Stillinger-Weber-like interactions, affects not only the role of local defects, but also the size of the plastic rearrangements, and the global stress-strain behavior. We study the effect of structure changes due to different quenching rates as well. We conclude the paper by a comparison between amorphous plasticity and the Peierls-Nabarro theory of plasticity in crystals.