Evaluating the harmonic approximation for the prediction of thermodynamic formation properties of solids

Thermodynamic stability is regularly quantified using the enthalpy of formation (Delta H-f) and the Gibbs free energy of formation (Delta(f)G). Although knowledge of these properties is crucial for many applications, literature data are often missing for many solids. We evaluate the harmonic approximation (HA) for the prediction of Delta H-f and Delta(f)G at constant volume and pressure, based on phonon calculations. Using density functional theory to carry out phonon calculations, we show that the HA excellently describes the temperature-dependence of Delta H-f and Delta(f)G for 14 compounds; mean absolute error (MAE) of 2.1 kJ.mol(-1) and 1.1 kJ.mol(-1), respectively, in the temperature interval 0-2500 K. Moreover, the performance of the HA was evaluated using computational data from the Materials Project database for 69 compounds yielding an MAE of 1.1 kJ.mol(-1) and 1.1 kJ.mol(-1), respectively, in the temperature interval 100-800 K. Very good performance is also observed for a number of additional compounds, including several hydrated salts, at 298.15 K. The model is subsequently applied to a number of phase-transition phenomena that demonstrate the strengths and weaknesses of the model. In addition, it is demonstrated that the HA model can provide quantitative performance that rivals that of the conventional quasi-harmonic approximation for the prediction of formation properties, at a significantly reduced computational effort (similar to 5-10 times faster).