My research applies multiscale quantum mechanical and classical modelling to material systems. In particular I am interested in the brittle fracture of semiconductor systems. When a brittle material is loaded to the limit of its strength, it fails by nucleation and propagation of a crack. The conditions for crack propagation are created by the concentration of a long-range stress field (illustrated in image, right) at an atomically sharp crack tip, creating a complex and strongly coupled multiscale system.

My PhD thesis reports the results of multiscale simulations of the brittle fracture of silicon on the (111) cleavage plane. The simulations are made possible by combining a quantum mechanical description of the processes taking place near the crack tip with a classical atomistic model that captures the long-range elastic relaxation. The Learn on The Fly technique is used to couple the quantum and classical models, allowing accurate quantum forces (red atoms in image, left) to be combined with classical forces (yellow atoms) using a simple adjustable potential to give stable dynamics.

Density Functional Theory simulations using the CASTEP code show the crack tip reconstruction process, with the 5-7 reconstruction forming dynamically prior to the onset of fracture and acting as a barrier to fracture.

Similar results are obtained with a variety of simpler tight binding models, in this case the Kwon TB potential.

In a thicker slab, the formation of the reconstruction across multiple layers can be observed. Here the QM force model is the DFTB tight binding parameterisation, and the reconstuction forms below the critical load prediction by continuum elasticity theory.