Advancing multiscale simulation methods
Most phenomena relevant for material science are best targeted by specific theoretical frameworks and simulation methods. For realistic description of material behavior, the department develops new schemes to couple elementary approaches and study their interplay in a systematic way.
The computational design of high strength steels such as FeMn alloys often faces a combination of challenges: (1) the treatment of chemical complexity, (2) the treatment of magnetic disorder, in particular, in the paramagnetic state, and (3) the treatment of structural defects. Moreover, the interplay of these degrees of freedom also needs to be accounted for. In this project we particularly focus on this kind of coupling of different degrees of freedom, since we believe it is decisive to understand some of the phenomena observed in FeMn alloys.
It is very challenging to simulate within DFT extreme electric fields (a few 1010
V/m) at a surface, e.g. for studying field evaporation, the key mechanism in atom probe tomography (APT). We have developed a straight-forward scheme to incorporate an ideal plate counter-electrode in a nominally charged repeated-slab calculation by means of a generalized dipole correction of the standard electrostatic potential obtained from fully periodic FFT.
In order to estimate the kinetics of thermally activated processes, one must determine the energy of the transition state. This transition state is a first-order saddle point on the potential energy surface, i.e., it is a maximum along the reaction coordinate, but a minimum with respect to all other directions in configurational space. We have developed an efficient Quasi-Newton algorithm to optimize the structure of the transition state.
Using technologically interesting examples, such as wurtzite surfaces, we develop a robust passivation scheme for density-functional-theory surface calculations of materials exhibiting spontaneous polarization. The novel approach enables computationally efficient and accurate surface electronic structure calculations.
We simulate the ionization contrast in field ion microscopy arising from the electronic structure of the imaged surface. For this DFT calculations of the electrified surface are combined with the Tersoff-Hamann approximation to electron tunneling. The approach allows to explain the chemical contrast observed for NiRe alloys.
This ERC-funded project aims at developing an experimentally validated multiscale modelling framework for the prediction of fracture toughness of metals.
The supercell approach allows to model defects with efficient periodic boundary models. By making the supercell sufficiently large, in principle, the limit of an single defect can be recovered. In practice, defect-defect interactions are still relevant for affordable system sizes. We demonstrate that empirical extrapolation has its limitations if the underlying physics is not taken into account or not even known.