Multiple mpirun's in one pbs job
For an example of multiple mpirun's in a single PBS job see: VASP example
Chaining pbs jobs
This is actually a pretty good primer on job chaining techniques. https://www.nics.tennessee.edu/computing-resources/running-jobs/job-chaining
If you combine the above two you can run many long trajectories or relaxations via backfilling without fragmenting the hpc environment and making highly parallel jobs difficult to launch.