The periodic and cluster approaches have been utilized for Fe–ZSM-5 and Fe–FER zeolites [41,42] and secondary numerical changes between the results have been only reported. Furthermore, cluster methods were widely employed to create a quality image at molecular level for NO and CO adsorption on several iron sites in MFI zeolite [43]. The adsorbed complexes have very similar energetic and geometrical properties on both 83T-large and 5T-small models [43]. It must be noted that DFT calculations on the periodic or cluster zeolite models usually give very similar reactivity trends [43–46]. So, a cluster modeling method (relatively a larger cluster) was used in order to pretend extra framework metal species of ZSM-12 zeolite in this study