Hard scaling challenges for ab initio molecular dynamics capabilities in NWChem: Using 100,000 CPUs per second

An overview of the parallel algorithms for ab initio molecular dynamics (AIMD) used in the NWChem program package is presented, including recent developments for computing exact exchange. These algorithms make use of a two-dimensional processor geometry proposed by Gygi et al. for use in AIMD algorithms. Using this strategy, a highly scalable algorithm for exact exchange has been developed and incorporated into AIMD. This new algorithm for exact exchange employs an incomplete butterfly to overcome the bottleneck associated with exact exchange term, and it makes judicious use of data replication. Initial testing has shown that this algorithm can scale to over 20,000 CPUs even for a modest size simulation.