Thomas Blesgen

Approximation of the electron density of Aluminium clusters in tensor-product format

The tensor-structured methods developed recently for the accurate calculation of the Hartree and the non-local exchange operators have been applied successfully to the ab initio numerical solution of the Hartree-Fock equation for some molecules. In the present work, we show that the rank-structured representation can be gainfully applied to the accurate approximation of the electron density of large Aluminium clusters. We consider the Tucker-type decomposition of the electron density of certain Aluminium clusters originating from finite element calculations in the framework of the orbital-free density functional theory. Numerical investigations of the Tucker approximation of the corresponding electron density reveal the exponential decay of the approximation error with respect to the Tucker rank. The resulting low-rank tensor representation reduces dramatically the storage needs and the computational complexity of the consequent tensor operations on the electron density. As main result, the rank of the Tucker approximation for the accurate representation of the electron density is small and only weakly dependent on the system size for the systems studied here. This shows good promise for resolving the electronic structure of materials using tensor-structured techniques.

On the competition of elastic energy and surface energy in discrete numerical schemes

The Γ-limit of certain discrete free energy functionals related to the numerical approximation of Ginzburg–Landau models is analysed when the distance h between neighbouring points tends to zero. The main focus lies on cases where there is competition between surface energy and elastic energy. Two discrete approximation schemes are compared, one of them shows a surface energy in the Γ-limit. Finally, numerical solutions for the sharp interface Cahn–Hilliard model with linear elasticity are investigated. It is demonstrated how the viscosity of the numerical scheme introduces an artificial surface energy that leads to unphysical solutions.

On the Allen—Cahn/Cahn—Hilliard system with a geometrically linear elastic energy

We present an extension of the Allen-Cahn/Cahn-Hilliard system which incorporates a geometrically linear ansatz for the elastic energy of the precipitates. The model contains both the elastic Allen-Cahn system and the elastic Cahn-Hilliard system as special cases and accounts for the microstructures on the microscopic scale. We prove the existence of weak solutions to the new model for a general class of energy functionals. We then give several examples of functionals that belong to this class. This includes the energy of geometrically linear elastic materials for D<3. Moreover we show this for D=3 in the setting of scalar-valued deformations, which corresponds to the case of anti-plane shear. All this is based on explicit formulas for relaxed energy functionals newly derived in this article for D=1 and D=3. In these cases we can also prove uniqueness of the weak solutions.

Discrete Free Energy Functionals for Elastic Materials with Phase Change

We discuss two different approaches related to Γ-limits of free energy functionals. The first gives an example of how symmetry breaking may occur on the atomistic level, the second aims at deriving a general analytic theory for elasticity on the lattice scale that does not depend on an explicitly chosen reference system.

A General Theory for Elastic Phase Transitions in Crystals

We derive a general theory for elastic phase transitions in solids subject to diffusion under possibly large deformations. After stating the physical model, we derive an existence result for measure-valued solutions that relies on a new approximation result for cylinder functions in infinite settings.