Axel Dian | Searcharxiv

arXiv2026

Free energies and optimal reaction coordinates via entropy production

We show through theory and numerical experiments that a straightforward calculation of entropy production on short molecular dynamics trajectories allows estimating free-energy barriers and identifying optimal reaction coordinates of activated processes. To this aim, we perform an analysis based on stochastic energetics on a set of trajectories relaxing towards equilibrium from a same initial configuration, projected on different putative coordinates. After demonstrating the approach on simple benchmarks, we show that it is possible to estimate the free-energy barrier of a complex high-dimensional system (carbon nanoparticles in water) and to rank the quality of order parameters, in agreement with the committor probability. The results shed light on the entanglement between the second law, free-energy landscapes, reaction coordinates and kinetic rates.

cond-mat.stat-mech↗

arXiv2025

Water is a superacid at extreme thermodynamic conditions

The chemical behavior of water under extreme pressures and temperatures lies at the heart of processes shaping planetary interiors, influences the deep carbon cycle, and underpins innovative high-temperature, high-pressure synthesis of materials. Recent experiments reveal that hydrocarbons immersed in ionized water under extreme conditions transform into heavy hydrocarbons and nanodiamonds. However, the chemistry of water at extreme conditions and its role in hydrocarbon condensation remains poorly understood. Here, using ab initio molecular dynamics simulations with enhanced sampling techniques and machine-learning interatomic potentials, we show that increasing pressure at high temperature induces water ionization, creating a superacid-like environment that favors the protonation of hydrocarbons into transient pentacoordinated carbonium ions like CH$_5^+$. These elusive intermediates release molecular hydrogen and yield highly reactive carbocations, driving hydrocarbon chain growth toward nanodiamonds. We demonstrate how the combination of water ionization and pressure-induced methane polarization leads to superacid-driven hydrocarbon chemistry, famously known at far milder conditions. Our findings reveal, for the first time, a superacid aqueous regime and establish the existence of superacid chemistry under extreme conditions. Moreover, they provide a unifying reaction network that explains chemical transformations in environments such as planetary interiors and high pressure, high temperature experiments.

physics.chem-ph↗