Load balancing of molecular dynamics simulation with NWChem

NWChem is a computational chemistry software suite developed for massively parallel computers in the W. R. Wiley Environmental Molecular Sciences Labo ratory at the U.S. Department of Energy's Pacific Northwest National Labora tory. This software integrates a range of modules for computational chemist ry applications, including classical molecular dynamics simulations and qua ntum mechanical calculations. This contribution provides details of the cla ssical molecular dynamics module and focuses on issues related to load bala ncing on massively parallel computers, in particular the ISM SPN and the Gr ay T3E (TM) as examples of distributed and shared memory massively parallel architectures. The implementation of the molecular dynamics module of NWCh em is based on a domain decomposition of the chemical system, faking advant age of the distribution of data to reduce the memory requirements and the l ocality of intermolecular interactions to reduce the communication requirem ents. This approach results in a more complex implementation because of the requirement of periodic atomic reassignments and the need for sophisticate d load-balancing techniques.