Software components for parallel multiscale simulation: an example with LAMMPS

Multiscale simulation is a promising approach for addressing a variety of real-world engineering problems. Various mathematical approaches have been proposed to link single-scale models of physics into multiscale models. In order to be effective, new multiscale simulation algorithms must be implemented which use partial results provided by single-scale software. This work considers aspects of software design for interfacing to existing single-scale simulation code to perform multiscale simulations on a parallel machine. As a practical example, we extended the large-scale atomistic/molecular massively parallel simulator (LAMMPS) atomistic simulation software to facilitate its efficient use as a component of parallel multiscale-simulation software. This required new library interface functions to LAMMPS that side-stepped its dependence on files for input and output and provided efficient access to LAMMPS's internal state. As a result, we were able to take advantage of LAMMPS's single-scale performance without adding any multiscale-specific code to LAMMPS itself. We illustrate the use of LAMMPS as a component in three different modes: as a stand-alone application called by a multiscale code, as a parallel library invoked by a serial multiscale code, and as a parallel library invoked by a parallel multiscale code. We conclude that it is possible to efficiently re-use existing single-scale simulation software as a component in multiscale-simulation software.