GPU-Acceleration of the WEST Code for Simulating Electronic Excitations in Large, Heterogeneous Materials

We present a massively parallel, GPU-accelerated implementation of many-body perturbation theory and time-dependent density-functional theory (TDDFT) in the WEST code (https://west-code.org) for the simulation of the excited states of large molecules and materials. Outstanding performance and scalability are achieved by employing a hierarchical parallelization strategy, overlapping communications with computations, and utilizing the near-sightedness principle in selected portions of the code. We show the results of large-scale calculations using full-frequency G0W0, the Bethe-Salpeter equation, quantum defect embedding theory, and TDDFT with hybrid functionals and analytical forces, for systems containing over a thousand atoms. Finally, we delve into our experience of switching GPU programming models to attain enhanced performance portability targeting the first exascale supercomputers deployed in the United States.