Ab initio molecular dynamics simulation technique is a promising tool to study various properties of vibrational spectra, entropy, free energy, etc. even at high temperatures. With the harmonic phonon model, all interatomic forces are treated as purely harmonic so that the equilibrium distance between atoms is independent of temperature. At the low temperature regime, this model, commonly used in most quantum chemistry codes, can approximately model vibrational properties and free energetics. At higher temperatures, however, the harmonic model can break down since it cannot explain thermal expansion, where the role of anharmonicity becomes very important in the description and interpretation of the system. To better describe properties of systems at high temperatures, the quasi-harmonic approximation can be. In this talk, I will present a few examples to show how the quasi-harmonic approximation is used to rigorously evaluate the vibrational density of states at elevated temperatures. From this, the quantitative evaluation of thermodynamic properties such as entropy, enthalpy, free energy, and equilibrium constants can then be inferred. A variety of systems ranging from ethanol-adsorption in zeolites (catalysis),[1] water/scCO2/mineral interfaces (carbon sequestration),[2] and Tc-incorporation in magnetite (nuclear remediation)[3] are used to illustrate the power of this approach.

[1] Alexopoulos, K.; Lee, M.-S.; Liu, Y.; Zhi, Y.; Liu, Y.; Reyniers, M.-F.; Marin, G. B.; Glezakou, V.-A.; Rousseau, R.; Lercher, J. A. J. Phys. Chem. C 120, 7172 (2016).
[2] Lee, M.-S.; McGrail, B. P.; Rousseau, R.; Glezakou, V.-G. Sci. Rep. 5, 14857 (2015).
[3] Lee, M.-S.; Um, W.; Wang, G.; Krug. A. A.; Lukens, W. W.; Rousseau, R.; Glazakou, V.-A. Nat. Commun. 7, 12067 (2016).