The flag -constant_seed allows use of a set RNG seed (the default is 1111111 or something similar).
The flag -jran allows you to pick a number for the RNG seed (-jran 429682, for example).
With MPI, the head node gets the seed specified by jran, and other processors get jran + their MPI rank. This ensures they all have independent trajectories, but also means you know which seed they started with.