CMSP Seminar (Atomistic Simulation Seminar Series) 15 January, 11:00AM, by Florian Pabst

[CMSP Seminars Secretariat]

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CMSP Seminar (Atomistic Simulation Seminar Series)
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*Wednesday, 15 January 2025, 11:00AM*
*/Luigi Stasi Seminar Room (Leonardo Building, 1st floor)/*/
/
/Zoom: 
https://zoom.us/meeting/register/lA5l9qH4ROKJ-97IiUUtLw<https://zoom.us/meeting/register/tJIrce-hrzktE9RzB_RZAY8u3Dp-NotIJNhG>
<https://zoom.us/meeting/register/lA5l9qH4ROKJ-97IiUUtLw> /


*Speaker: * Florian Pabst (SISSA)
*Title: *Molecular Dynamics of Liquids: From Experiments to Simulations

The rotational dynamics of molecules in the liquid phase can be 
routinely studied from the boiling point down to the glass transition 
temperature using dielectric spectroscopy, which spans 18 orders of 
magnitude in frequency. In recent years, depolarized light scattering 
has emerged as a competitive technique, offering comparable frequency 
range and resolution. Dielectric spectroscopy probes the permanent 
electric dipole moments of molecules, while light scattering is 
sensitive to polarizability tensors. Consequently, these methods provide 
complementary insights when their spectra are compared. In this talk, I 
will demonstrate how this dual approach can disentangle various dynamic 
contributions in ionic liquids, where conductivity complicates the 
interpretation of dielectric spectra [1]. Additionally, I will show that 
orientational cross-correlations between neighboring molecules dominate 
the dielectric response in hydrogen-bonded or highly polar substances 
but are largely absent from light scattering spectra [2].
Since both techniques access only the macroscopic response of the 
sample, microscopic interpretations of molecular dynamics are often 
ambiguous. Instead, ab initio simulations enable a detailed 
molecular-level understanding and simultaneously provide both dielectric 
and light scattering spectra. In the second part of my talk, I will 
present how deep neural networks, trained to reproduce interatomic 
forces, molecular dipoles, and polarizability tensors, yield spectra 
that align remarkably well with experimental results. This approach 
provides answers to questions such as: "Why does the dielectric 
absorption peak of water shift to higher frequencies with added salt, 
indicating faster dynamics, while increased viscosity suggests slower 
dynamics?" and "What causes the dramatic slowdown of molecular motions 
when approaching the glass transition temperature?" [3].

[1] Pabst et al., J. Phys. Chem. Lett. 10, 9, 2030 (2019)
[2] Pabst et al., J. Phys. Chem. Lett. 12, 14, 3685 (2021)
[3] Pabst et al., arXiv:2408.05528 (2024)

http://www.ictp.it/research/cmsp.aspx

The Abdus Salam International Centre for Theoretical Physics
https://www.ictp.it/

    
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