# Publications

## Atomistic aspects of fracture

E. Bitzek, J. R. Kermode and P. Gumbsch, Atomistic aspects of fracture, Int. J. Fract. (2015), doi: 10.1007/s10704-015-9988-2

Any fracture process ultimately involves the rupture of atomic bonds. Processes at the atomic scale therefore critically influence the toughness and overall fracture behavior of materials. Atomistic simulation methods including large-scale molecular dynamics simulations with classical potentials, density functional theory calculations and advanced concurrent multiscale methods have led to new insights e.g. on the role of bond trapping, dynamic effects, crack- microstructure interactions and chemical aspects on the fracture toughness and crack propagation patterns in metals and ceramics. This review focuses on atomistic aspects of fracture in crystalline materials where significant advances have been achieved over the last ten years and provides an outlook on future perspectives for atomistic modelling of fracture.

## Energy conserving, self-force free Monte Carlo simulations of semiconductor devices on unstructured meshes

M. Aldegunde and K. Kalna, "Energy conserving, self-force free Monte Carlo simulations of semiconductor devices on unstructured meshes", Comput. Phys. Commun. **189**, pp. 31-36 (2015), doi:10.1016/j.cpc.2014.11.020

Unphysical self-forces resulting from the particle–mesh coupling occur when ensemble Monte Carlo simulations of semiconductor devices use an unstructured mesh to describe device geometry. We report on the development of a correction to the driving electric field on arbitrary meshes and show that self-forces can be virtually eliminated on a finite element mesh at a small additional computational cost. The developed methodology is included into a self-consistent 3D finite element Monte Carlo device simulator. We show the efficiency of the method simulating an isolated particle and obtaining kinetic energy conservation down to a magnitude of 10^{−10} meV. The methodology is later applied to a FinFET simulation to show what impact can be expected from the self-forces using traditional electric field interpolation strategies. We find that for a large enough ensemble of particles, the impact of self-forces on the final I_{D}–V_{G} is almost negligible.

## Accuracy of buffered-force QM/MM simulations of silica

Anke Peguiron, Lucio Colombi Ciacchi, Alessandro De Vita, James R. Kermode and Gianpietro Moras, Accuracy of buffered-force QM/MM simulations of silica**, **J. Chem. Phys., **142**, 064116 (2015), doi: 10.1063/1.4907786

We report comparisons between energy-based quantum mechanics/molecular mechanics (QM/MM) and buffered force-based QM/MM simulations in silica. Local quantities—such as density of states, charges, forces, and geometries—calculated with both QM/MM approaches are compared to the results of full QM simulations. We find the length scale over which forces computed using a finite QM region converge to reference values obtained in full quantum-mechanical calculations is ∼10 Å rather than the ∼5 Å previously reported for covalent materialssuch as silicon. Electrostatic embedding of the QM region in the surrounding classical point charges gives only a minor contribution to the force convergence. While the energy-based approach provides accurate results in geometry optimizations of point defects, we find that the removal of large force errors at the QM/MM boundary provided by the buffered force-based scheme is necessary for accurate constrained geometry optimizations where Si–O bonds are elongated and for finite-temperature molecular dynamics simulations of crack propagation. Moreover, the buffered approach allows for more flexibility, since special-purpose QM/MM coupling terms that link QM and MM atoms are not required and the region that is treated at the QM level can be adaptively redefined during the course of a dynamical simulation.

## P. Brommer and D. Quigley, 2014 J. Phys.: Condens. Matter 26 485501

Peter Brommer and David Quigley: **Automated effective band structures for defective and mismatched supercells. ***J. Phys.: Condens. Matter* **26** 485501 (2014). doi: 10.1088/0953-8984/26/48/485501

In plane-wave density functional theory codes, defects and incommensurate structures are usually represented in supercells. However, interpretation of *E* versus **k** band structures is most effective within the primitive cell, where comparison to ideal structures and spectroscopy experiments are most natural. Popescu and Zunger recently described a method to derive effective band structures (EBS) from supercell calculations in the context of random alloys. In this paper, we present bs_sc2pc, an implementation of this method in the CASTEP code, which generates an EBS using the structural data of the supercell and the underlying primitive cell with symmetry considerations handled automatically. We demonstrate the functionality of our implementation in three test cases illustrating the efficacy of this scheme for capturing the effect of vacancies, substitutions and lattice mismatch on effective primitive cell band structures.

## P. Brommer et al. 2014 Phys. Rev. B 90 134109

Peter Brommer, Laurent Karim Béland, Jean-François Joly, and Normand Mousseau: **Understanding long-time vacancy aggregation in iron: A kinetic activation-relaxation technique study. ***Physical Review B ***90**, 134109 (2014). doi:10.1103/PhysRevB.90.134109

Vacancy diffusion and clustering processes in body-centered-cubic (bcc) Fe are studied using the kinetic activation-relaxation technique (k-ART), an off-lattice kinetic Monte Carlo method with on-the-fly catalog building capabilities. For monovacancies and divacancies, k-ART recovers previously published results while clustering in a 50-vacancy simulation box agrees with experimental estimates. Applying k-ART to the study of clustering pathways for systems containing from one to six vacancies, we find a rich set of diffusion mechanisms. In particular, we show that the path followed to reach a hexavacancy cluster influences greatly the associated mean-square displacement. Aggregation in a 50-vacancy box also shows a notable dispersion in relaxation time associated with effective barriers varying from 0.84 to 1.1 eV depending on the exact pathway selected. We isolate the effects of long-range elastic interactions between defects by comparing to simulations where those effects are deliberately suppressed. This allows us to demonstrate that in bcc Fe, suppressing long-range interactions mainly influences kinetics in the first 0.3 ms, slowing down quick energy release cascades seen more frequently in full simulations, whereas long-term behavior and final state are not significantly affected.