Nowadays density functional theory (DFT) methods have been very popular in investigating various chemical problems, due to the lower cost of computational time in dealing with electron correlation problem. However, the first thing one should do is to select the most suitable DFT method for the system and problem studied. Our recent studies reveal that electron correlation in tetrazoles is greater. To the best of our knowledge, no papers concerning investigations on tetrazoles using DFT approaches have been published. To select the appropriate DFT method for tetrazole, in this paper we employed various kinds of DFT methods on tautomerization of tetrazole. The results show that the optimized geometries and calculated energetic with hybrid DFT B3LYP at 6-31G^* level are in best agreement with the high level ab initio's results. In addition, The IR frequencies from B3LYP/6-31G^* are very close to the corresponding frequencies obtained from MP2/6-311G^*^*. The calculated thermodynamic properties with the unscaled frequencies from B3LYP/6-31G^* agree with the experimental ones excellently. The above facts show that hybrid B3LYP- DFT method is the first choice for investigating tetrazoles.

Key words: TETRAZOLE, ISOMERIZATION, ACTIVATION ENERGY, INFRARED SPECTROPHOTOMETRY, THERMODYNAMIC PROPERTIES