GPU-Accelerated Solution of the Bethe-Salpeter Equation for Large and Heterogeneous Systems

We present a massively parallel, GPU-accelerated implementation of the Bethe-Salpeter equation (BSE) in the WEST code [1,2] for the calculation of the vertical excitation energies (VEEs) and optical absorption spectra of condensed and molecular systems, starting from single-particle eigenstates obtained within density functional theory. The algorithms adopted here circumvent the slowly converging sums over empty and occupied states and the inversion of large dielectric matrices. We achieve computational savings by exploiting the nearsightedness of the density matrix of semiconductors and insulators, and we scale our calculations to thousands of GPUs with a hierarchical loop- and data-distribution strategy. We demonstrate the efficacy of our method by computing the VEEs of several spin defects in wide band-gap materials, showing that supercells with up to 1000 atoms are necessary to obtain converged results. We also discuss the accuracy of the conventional formalism that solves the BSE with truncated sums over empty and occupied states. We then apply our GW-BSE implementation to a diamond lattice with 1727 atoms to study the symmetry breaking of triplet states caused by the interaction of a point defect with an extended line defect.

[1] arXiv:2409.15116

[2] https://west-code.org