First-principles simulations of electronic excitations and real-time dynamics on high-performance super computers*

Excited electronic states and their ultrafast dynamics are foundational to how we interact with or probe many materials. Thanks to advanced experimentation, electronic excitations are accessible with high accuracy and time resolution, however, solid theoretical understanding is crucial for a detailed interpretation of experimental results. To this end, first-principles electronic-structure methods, such as many-body perturbation theory and time-dependent density functional theory, are powerful tools that provide highly accurate insight.

In this talk, I will discuss recent examples where we used real-time time-dependent density functional theory and Ehrenfest dynamics to study non-equilibrium electron dynamics coupled to classical ions. We performed simulations for highly energetic projectile ions impacting semiconductors and metals, that, however, suffer from high computational cost owing to small time steps during numerical integration. Massive parallelization and high-performance super computers such as Blue Waters and ALCF are needed, motivating our recent work on evaluating efficient and accurate numerical integrators within the Qb@ll code, e.g. using the PETSc library.

I will also discuss examples for using many-body perturbation theory to predict photon absorption and to establish the connection between structural and optical properties of semiconductors and their nanocrystals. In this context, the accurate computation of exciton binding energies is particularly demanding because it relies on eigenvalues of large dense matrices. I will illustrate how fast iterative eigensolvers help us tackle these problems on Blue Waters and broaden the applicability of many-body perturbation theory towards more diverse material systems.