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Theme 1 colloquium by dr. Daria Galimberti: 'Accurate Vibrational Free energy from Anharmonic Vibrations computed by DFT-MD simulations: the absorption of alkanes in a H-CHA zeolite test case' (Lecture)

Date
Tuesday 2 March 2021Add to my calendar
Time
from 16:00
Location
zoom
Speaker
dr. Daria Galimberti (Theoretical and Computational Chemistry)
Description

Statistical thermodynamics is a powerful tool to typically evaluate rate, equilibrium constants and changes in free energies in complex molecular systems. One important application is in heterogeneous catalysis where one wants to get chemically accurate free energies for all elementary steps such as adsorption, chemical transformation, and desorption.

While sampling the free energy surface by Molecular Dynamics (MD) or Monte Carlo simulations requires respectively long simulations and/or millions of energy calculations, vibrational density of states (VDOS) based on velocity correlation functions are converged within rather short MD trajectories of roughly 10 ps time-scales. I will present an MD based theoretical methodology that allows to compute in automatized and reliable ways free energies of absorption from vibrational partition functions based on the VDOS signal. Molecular Dynamics simulations naturally capture anharmonic effects and large amplitude motions of the vibrations that have shown in the past to give non-negligible contributions to the absorption free energies [1]. The required VDOS for the prediction of the vibrational partition function must contain vibrational fundamental bands only, while, by construction, the VDOS computed from MD trajectories includes also overtones and combination bands. I will show how the power spectra of the vibrational mode-projected velocity-autocorrelation functions allows to fulfill these requirements.

The results of our methodology will be critically assessed by comparing the free energy of absorption of small alkanes in H-CHA zeolite with the data obtained with other theoretical methodology [1] and experiments.

[1] Piccini, G. et al. J. Phys. Chem. C 2015, 119, 6128-6137

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