Being Fast without Cutting Corners in Spectroscopy and Free Energy Calculations

Daria Ruth Galimberti
Institute for Molecules and Materials, Radboud University Nijmegen, The Netherlands

Thuesday 17 June 2025, 11:00am
Lieu: Bibiothèque du LCT, Couloir 12-13, 4^ème étage, Campus Pierre et Marie Curie

I will present examples of the recent developments done in my group to compute accurate vibrational spectra and reaction free energies for complex molecular systems where standard methods fail or are not feasible due to the computational cost.

In particular, I will illustrate our recently developed MD-based theoretical methodology (DOS-P) to compute chemically accurate (anharmonic) free energies. While sampling the free energy surface by Molecular Dynamics (MD) or Monte Carlo simulations requires respectively long simulations (50 ps up to 1ns) and/or millions of energy calculations, DOS-P allows obtaining well-converged enthalpies, entropies, and free energies of adsorption from vibrational partition functions based on the (anharmonic) VDOS signal using a set of short trajectories (maximum 3ps). The reduced computational cost of DOS-P enables the use of QM:QMMD simulations in which an accurate high-level quantum mechanics description is used for the reactive sub-system, together with a low-level periodic quantum mechanics description for the rest. In the talk, I will critically compare the adsorption geometry, enthalpy, entropies, and free energy computed at the B3LYP+D2:PBE+D2 level of theory with the PBE+D2 data and the experimental values, using the adsorption of ethanol in H-MFI zeolite as a test case.

In the second part of the talk, I will discuss our Activity Weighted Velocities (AWV) method to integrate classical trajectories with the accuracy of hybrid Density Functional Theory (DFT) Raman activities. We demonstrate that this combination effectively computes accurate Low-Frequency/THz Raman spectra and offers a way to predict the Raman activity of Thermally Disordered Systems and Entropically Stabilized Phases. As an example, I will show how with this method we have been able to disclose the nature of the 4-DBpFO thermosalient (or ``jumping'') crystal phase ~1.2 THz "gateway" mode using low-frequency Raman Spectroscopy (LFRS). We explore and reveal the roles of anharmonicity, thermal disorder, and volume fluctuations in determining the THz spectral shape. The anharmonic couplings and cell fluctuations appear to be more than just innocent spectators, and indeed, the intermolecular correlations for this mode involve multiple cells.

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References :
[1] D. R. Galimberti, J. Sauer, J. Chem. Theory Comput. 2021, 17, 5849.
[2] D. Kumar, J. Sauer, A. Airi, S. Bordiga, D. R. Galimberti, Phys. Chem. Chem. Phys. 2025, 27, 550.
[3] S. Ghasemlou, X. Li, D. R. Galimberti, T. Nikitin, R. Fausto, J. Xu, S. Holleman, T. Rasing, H. M. Cuppen, Proc. Natl. Acad. Sci. U.S.A. 2024, 121, e2408366121.