def fix_iso(job):
    """
    Add couple xyz to the fix_ensemble inside LAMMPS

    Args:
        job (LAMMPS): Lammps job object

    Returns:
        LAMMPS: Return updated job object
    """
    job.input.control["fix___ensemble"] = (
        job.input.control["fix___ensemble"] + " couple xyz"
    )
    return job