Chemical Reactions: Thermochemical Calculations

This chapter provides an introduction to the calculation of thermochemical data for chemical reactions using quantum chemical methods. The basic procedure is first described, namely, obtaining molecular structures and electronic energies of reactants and products, followed by vibrational frequency calculations and evaluation of thermal corrections. Since it is harder to obtain a given accuracy for some types of reactions than others, some discussion is provided on classes of reactions (e.g., isodesmic reactions) for which a given accuracy is easier to achieve than for a general reaction. Three examples illustrate different aspects of thermochemical calculations. The first example, the formation of ammonia from its elements, illustrates a variety of basis set and correlation effects on calculated data. The second example is concerned with calculations on small fluorine-oxygen species and a systematic side-by-side comparison of coupled-cluster and density-functional methods, including the use of isodesmic reactions. The third example describes the use of high-level coupled-cluster calculations to predict the standard enthalpy of formation of S(OH)₂.