The effect of cooling rate and force field on the glass transition temperature of atmospheric molecules
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Date
2025-07-07
Authors
Τσατσούλης, Αλέξανδρος
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Abstract
This diploma thesis is part of a collaboration between the Laboratory of Statistical Thermodynamics and Macromolecules (LSTM), led by Prof. Vlasis Mavrantzas, and the Laboratory of Air Quality Studies (LAQS), led by Prof. Spyros Pandis. The aim of this work is to employ the strong capabilities of molecular simulations to the study of atmospheric aerosols. This study follows a previous collaborative work where Molecular Dynamics (MD) simulations were performed for a series of atmospherically relevant organic compounds in order to determine their glass transition temperature (Tg). The objective of this study is to investigate how the choice of cooling rate and the selection of the force field influence the predicted Tg value. The molecule of 1,2,6-hexanetriol was used for the cooling simulations. Four variations of the Optimized Potential for Liquid Simulations-All Atom (OPLS-AA) force field and a cooling rate of 20 K/2 ns were implemented, and the MD simulations were performed in the isobaric-isothermal (NPT) statistical ensemble. Three different properties of the system⎯density, enthalpy and non-bonded energy were used for the determination. Finally, a slower cooling rate of 20 K / 20 ns was used on one of the force field variations.
This study demonstrated that the cooling rate has a significant impact on the determined Tg, with the slower cooling rate producing more consistent results that are closer to the experimental values. The selection of force field was found equally important for the method’s accuracy: versions with outdated parameters or non-personalized charges failed to adequately capture the behavior of the system. Among the three properties used, density exhibited a more physically realistic depiction with the decrease of temperature, while enthalpy introduced Tg values that were both near the experimental data and consistent across the different force fields. Tg values based on non-bonded energy showed greater variability across different force fields but were marginally reduced when a slower
cooling rate was applied.
All in all, the findings suggest that when dealing with systems of atmospheric organic compounds—where complex interactions require detailed molecular representation—it is more suitable to use force fields such as the OPLS/2020 force field combined with Localized Bond-Charge Corrected
(LBCC) charges, which incorporate molecule-specified parameters instead of universal ones. Additionally, slow cooling rates of at least 20 K per20 ns are recommended for reliable Tg determination in such systems.
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Keywords
Molecular dynamics, Glass transition temperature, Atmospheric studies