OpenMP doesn’t require a recompile, just set environment variable $OMP_NUM_THREADS (normally done in the Cylc task configuration) and make sure the PBS number of CPUs is (MPI ranks) * (OMP threads)
NPROC restriction is because there’s a minimum grid size for each process, e.g. halo of cells going to the north MPI rank can’t overlap with the halo of cells going to the south MPI rank.