In some edge cases a user may want to only run 1 MPI tasks on a node which contains 8 cores.

Example 1:

To do this change your script to the following:

#!/bin/sh
#SBATCH -N 4            # ask for 4 nodes
#SBATCH -t 1-03:00:00   # 1 day and 3 hours
#SBATCH -p compute      # partition name
#SBATCH -U chemistry    # your project name - contact Ops if unsure what this is
#SBATCH -J my_job_name  # sensible name for the job

mpirun -npernode 1 ./cpi.x

The above assumes that openmpi is being used and will ask for 4 nodes but launch 1 process per node.

Example 2:

example is as follows:

#!/bin/sh
#SBATCH -N 8            # ask for 8 nodes
#SBATCH -t 1-03:00:00   # 1 day and 3 hours
#SBATCH -p compute      # partition name
#SBATCH -U chemistry    # your project name - contact Ops if unsure what this is
#SBATCH -J my_job_name  # sensible name for the job

mpirun -npernode 4 ./cpi.x

This gives you 8 nodes and runs 4 mpi process on each node (i.e. 32 mpi processes in total)