Performance of DFT, MP2, and composite ab initio methods for the prediction of enthalpies of formations of CHON compounds using isodesmic reactions

This paper assesses the performance of quantum chemical models with regard to the calculation of enthalpy of formation of CHON molecules using isodesmic reactions. The high accuracy of prediction of enthalpy of formation of CHON compounds can be achieved by the combination of isodesmic reaction scheme with composite ab initio methods. The best composite methods, such as G4, G4MP2, CBS-QB3, and CBS-APNO, can attain near chemical accuracy or better depending on reaction type. Other composite methods yield the same accuracy only in conjunction with isodesmic or homodesmotic reactions. The DFT and MP2 methods lead to good results if they are used with homodesmotic reactions. For CHO compounds, all composite methods demonstrate the high accuracy not only with isodesmic and homodesmotic reactions, but also with isogyric reactions. Moreover, the DFT and MP2 methods can also yield a high accuracy estimate of the enthalpy of formation when they are used with isodesmic or homodesmotic reactions. Four composite methods (G4, G3, CBS-APNO, and CBS-QB3) employed in conjunction with 45 isogyric reactions yield the enthalpy of formation of hydrazine (97.7 ± 2.0 kJ/mol) in agreement with the best approximating to CCSD(T)/CBS energy, thus calling into question the usually quoted experimental value of 95.5 kJ/mol.