Speaker: Johannes Dietschreit, University of Vienna

Understanding chemical reactions at the molecular level often relies on projecting dynamics onto a few collective variables (CVs) that capture the key, slow degrees of freedom. A widely used construct in this context is the potential of mean force (PMF)—a free energy-like profile derived from the marginal Boltzmann distribution along a CV. While commonly interpreted as an analogue of the minimum energy path, the PMF depends on the chosen CV function and can potentially assign different values to identical configurations in Cartesian space, potentially making such interpretations inconsistent.

While neglecting this property can be a valid approximation for some reactions, it can lead to substantial errors in estimating reaction energetics. As an alternative, we derive exact expressions for the free energy, internal energy, and entropy profiles as functions of the CV that are gauge-invariant [1]. This leads to simple and consistent definitions of reaction [2] and activation [3] energies and entropies that, unlike the PMF, account for particle masses, which is crucial for the correct description of kinetics.

We illustrate how this framework enables the extraction of quantitative insights from entropy and energy profiles in realistic physicochemical processes, including intramolecular organic reactions.

[1] Dietschreit, J. C. B.; Diestler, D. J.; Gómez-Bombarelli, R. Entropy and Energy Profiles of Chemical Reactions. J. Chem. Theory Comput. 2023, 19(16), 5369-5379
[2] Dietschreit, J. C. B.; Diestler, D. J.; Ochsenfeld, C. How to obtain reaction free energies from free-energy profiles. J. Chem. Phys. 2022, 156(11), 114105
[3] Dietschreit, J. C. B.; Diestler, D. J.; Hulm, A.; Ochsenfeld, C.; Gómez-Bombarelli, R. From free-energy profiles to activation free energies. J. Chem. Phys. 2022, 157(8), 084113