def get_default_options(max_num_machines=1, max_wallclock_seconds=1800,
    with_mpi=False):
    """Return an instance of the options dictionary with the minimally required parameters for a `CalcJob`.

    :param max_num_machines: set the number of nodes, default=1
    :param max_wallclock_seconds: set the maximum number of wallclock seconds, default=1800
    :param with_mpi: whether to run the calculation with MPI enabled
    """
    return {'resources': {'num_machines': int(max_num_machines)},
        'max_wallclock_seconds': int(max_wallclock_seconds), 'withmpi':
        with_mpi}
