  Reactive molecular dynamics simulations are computationally demanding.
Reaching spatial and temporal scales where interesting scientific phenomena can
be observed requires efficient and scalable implementations on modern hardware.
In this paper, we focus on optimizing the performance of the widely used
LAMMPS/ReaxC package for multi-core architectures. As hybrid parallelism allows
better leverage of the increasing on-node parallelism, we adopt thread
parallelism in the construction of bonded and nonbonded lists, and in the
computation of complex ReaxFF interactions. To mitigate the I/O overheads due
to large volumes of trajectory data produced and to save users the burden of
post-processing, we also develop a novel in-situ tool for molecular species
analysis. We analyze the performance of the resulting ReaxC-OMP package on
Mira, an IBM Blue Gene/Q supercomputer. For PETN systems of sizes ranging from
32 thousand to 16.6 million particles, we observe speedups in the range of
1.5-4.5x. We observe sustained performance improvements for up to 262,144 cores
(1,048,576 processes) of Mira and a weak scaling efficiency of 91.5% in large
simulations containing 16.6 million particles. The in-situ molecular species
analysis tool incurs only insignificant overheads across various system sizes
and run configurations.
