Improving the Scalability of Condensed-Phase Hybrid Density Functional Theory: Computation, Communication, and Load Balancing

Ab initio molecular dynamics (AIMD) simulations at the hybrid density functional theory (DFT) level provide a semi-quantitative description of complex condensed-phase systems such as molecular liquids and crystals. For finite-gap systems, we have developed a linear-scaling and formally exact algorithm for computing the exact exchange interaction in real space based on a localized representation of the occupied orbitals (e.g., maximally localized Wannier functions). Although a massively parallel implementation of this algorithm in Quantum ESPRESSO already enables hybrid DFT based AIMD simulations of condensed-phase systems containing 500-1000 atoms, we have identified three nearly equal contributions to the walltime cost of this approach: computation events, communication overhead, and processor idling due to workload imbalance. In this work, we present a three-pronged strategy that we have employed to attack these contributions and reduce the overall walltime cost by approximately an order of magnitude.