def METHOD_NAME(field,data):
    ad = ds.all_data()
    metals = np.zeros(len(ad[("PartType4","Metallicity_00")]))
    for i in range(0,4):
        metals += ad[("PartType4","Metallicity_0%s"%i)]
    #the metallicity in romeel/benopp simulations needs to be
    #multiplied by a metallicity factor of 0.0189/0.0147 as
    #stolen from bobby thompson's pygadgetreader
    metals *= 0.0189/0.0147
    return data.ds.arr(metals,'code_metallicity')