PBS Tricks

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.