NWChem is the software package for computational chemistry on massively par
allel computing systems developed by the High Performance Computational Che
mistry group for the Environmental Molecular Sciences Laboratory. The softw
are provides a variety of modules for quantum mechanical and classical mech
anical simulation. This article describes the design of the molecular dynam
ics simulation module, which is based on a domain decomposition, and provid
es implementation details on the data and communication structure and how t
he code deals with the complexity of atom redistribution and load balancing
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