Prediction of the Enthalpy of Formation by DFT (Density Functional Theory) Calculations

Enthalpies of formation (EoF) are important quantities in the determination of the performance data of explosive materials. Not always they are available. Several attempt and procedures exist to calculate them. There are structure-property correlation methods, additivity methods and quantum mechanical methods. For the prediction of the standard enthalpies of formation (EoF) of organic compounds, in our case for energetic materials and their components, semi-empirical computational methods based on quantum-mechanical (QM) methods were selected to calculate these values with sufficient accuracy. For the QM calculations, we have used the program suite Gaussian16 Rev. C.1.1 from company GaussianTM Inc. The first step is the calculation of the standard enthalpy of formation in the gas phase. Two of many methods are to calculate the EoF via the enthalpies of reaction: (1) from elements to molecule; (2) atomization, from molecule to elements. For both methods, correction terms were developed: for (1) see Politzer and for (2) see Rice. Further, the so-called compound (composite or multi-level) methods as CBS-QB3, G4, G4-MP2, W1-BD from GaussianTM Inc are used for this purpose, which have higher accuracy than the earlier DFT-methods used by Politzer and Rice. The second step is to get the enthalpies of formation of the condensed phases starting from the gas data. For this the enthalpy of vaporization must added in case of a liquid and the enthalpy of sublimation in case of a solid. To obtain these data two empirical formulas are given to calculate them on the base of the electrostatic potential and the molecular surface, whereby the software given in can be used to get the necessary parameters. Rice has adopted the Politzer method but used another set of molecules to determine the empirical fit parameters. Mainly CHNO molecules are of interest, but possibilities exist to include the elements S, P, F, Cl, and some others can be considered as Na, Mg, Al, Si. Examples are presented, and the data compared to experimentally determined values of EoF.