Towards Exascale Electronic Structure and Quantum Transport Calculations

The development of robust, adaptive software and algorithms that can fully exploit exascale capabilities and future computing architectures is critical to designing advanced materials and devices with targeted properties. We have developed an open-source code that discretizes the DFT equations on real-space grids that are distributed over the nodes of a massively parallel system via domain decomposition. Multigrid techniques are used to dramatically accelerate convergence while only requiring nearest-neighbor communications. The real-space multigrid (RMG) code achieves full plane wave accuracy and scales from desktops and clusters to supercomputers consisting of ~200k cores and 20k GPUs, including the Cray XE-XK systems and the new IBM-NVIDIA pre-exascale Summit. Multilevel parallelization with MPI, threads and/or Cuda/HIP programming enables adaptation to future exascale supercomputers. RMG is distributed via www.rmgdft.org, with over 3,800 downloads do date. Advanced functionalities are provided through interfaces to other codes, including QMCPACK, BerkeleyGW, Phonopy, and ALAMODE. RMG is also being used for high throughput vibrational analysis of large systems at the Spallation Neutron Source. We will also describe the non-equilibrium Green’s function module based on variationally-optimized localized orbitals, by which quantum transport properties can be studied for devices containing tens of thousands of atoms with full DFT accuracy. For a system with ten thousand atoms, our initial implementation scales linearly from 100 to 1000 nodes on Summit, already gaining ~4x speed-up from GPUs over CPU-only calculations. Several applications will be described as time permits, including a nanocircuit that could potentially enable electrical sequencing of DNA, and a novel experimentally realizable graphene-nanoribbon-based negative differential resistance device.
In collaboration with E. L. Briggs, W. Lu, Z. Xiao, Y. Li, J. Zhang, Y. Cheng, A.J. Ramirez-Cuesta and M. Hodak.