NERSC Initiative for Scientific Exploration (NISE) 2010 Awards

Pushing the limits of the GW/BSE method for excited-state properties of molecules and nanostructures

Jack Deslippe, University of California, Berkeley

Associated NERSC Project: Computation of Electronic Properties of Materials from First Principles and on surfaces (mp149), Principal Investigator: Steven Louie, Berkeley Lab

Accurate description of interaction of light with matter is important for developing new materials used in photovoltaic applications. This research focuses on improving the description of the interaction of light with molecules and nano-systems that are likely to be the building blocks of future photovoltaic devices. Additionally, this research will quantitatively test the applicability of many-body physics' approaches to the study of systems traditionally described using quantum chemistry approaches.

The development of light-weight protection materials for military applications is urgently needed for a nation fighting unconventional war. Boron-rich boron carbide (B4+xC) is a leading candidate for such promising materials. However, in spite of many years of research and development, full utilization of boron carbide as protection material has not been fully realized. Ballistic tests indicate that this material lost its shear strength when the pressure exceeds its Hygoniot elastic limit (HEL) of about 20 GPa. Postmortem analysis of dynamically deformed samples indicates boron carbide undergoes shock-induced amorphization when subject to high velocity impact pressure. The mechanism of amorphization and the concomitant plastic behavior of B4+xC under ballistic loading and unloading are not understood.

We plan to investigate the structural deformation and the changes in elastic properties of boron carbide using large-scale ab initio simulations at the atomic level based on density functional theory. Boron carbide has a unique structure consisting of icosahedral B11C unit and a three atom C-B-C chain in the axial direction of the rhombohedral cell. It is characterized by strong intra-icosahedral and inter-icosahedral covalent and three-center bonds. To understand the amorphization and deformation behavior of boron carbide under pressure, extensive simulations using large supercells of at least several hundred atoms per cell must be used which are computationally very demanding. The simulations entail theoretical experiments of applying, step-by-step, uniaxial pressure in the axial direction to the rhombohedral supercells up to 50% of volume reduction. At each level of strain, the atomic structural evolution, the elastic modulus, the stress level, the electronic structure and the inhomogeneous localization of the amorphous zone will be investigated in order to understand the amorphization process and to find ways to mitigate structural softening of the material beyond HEL.

The methods and the computational codes developed using NERSC supercomputing facility under NISE program will also be applied to other potential candidates of light-weight protection materials for both military and civilian use. Based on systematic simulations and extensive data collected, a comprehensive database for mechanical properties of boron rich and other compounds under extreme conditions will be generated that can be used for future modeling at the macro-scale.