Starting from AMS2019, ReaxFF has become multithreaded. If the user wishes to limit ReaxFF execution to one core they should set the OMP_NUM_THREADS environment variable to 1. This variable is set automatically to a proper value when using Intel-MPI binaries with a supported batch system on Linux (SLURM, PBS, etc.). In all other cases ReaxFF will try to use all cores available in the machine.