CHE 598 Seminar: Anharmonic Effects on Material Properties at High Temperatures From Ab Initio Molecular Dynamics Simulations

About the event

Presenter:

Mal-Soon Lee, Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory (PNNL).

Biography:

Mal-Soon Lee is a senior research scientist who has been working in the field of computational physics and chemistry with an emphasis on studying the phase behavior and reactivity at complex interfaces using a variety of statistical mechanical tools. Her areas of application include studies of catalysis (thermo- and electro-), surface science, polymer, CO2 sequestration, nuclear waste disposal, thermoelectric materials, clusters, and high-pressure physics. To understand the entropy and enthalpy components of reactivity of these heterophase materials including confined materials such as zeolites or solid/liquid interfaces, large-scale high performance computing techniques, such as classical and ab initio molecular dynamics simulations and/or ab initio electronic structure calculations, are employed. With these simulations she applies statistical mechanical techniques to calculate various properties such as reaction free energies and convoluted enthalpies/entropies, spectroscopic properties, such as IR or X-ray spectra which can be directly compared with experimental observations. In her career she has developed a variety of codes to calculate structural, energetic, and spectroscopic properties at elevated temperature and/or pressure conditions.

Abstract:

Ab initio molecular dynamics simulation technique is a promising tool to study various properties of vibrational spectra, X-ray spectra, free energy, entropy, 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 employed. In this presentation, I will discuss some examples of adsorption of molecules on zeolites (catalysis) and electrocatalytic conversion of biomass derived organics at solid/liquid interface (electrocatalysis) 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. In addition, I will present our recent study of structural properties and composition of active sites of Cu-Al-oxo clusters on Cu-MOR obtained from ab initio molecular dynamics (AIMD), simulated ensemble averaged Cu K- and L3-edge X-ray absorption near edge structure