On the connections between microcanonical and isothermal–isobaric ensemble dynamics in liquids

A new approach for calculating activation energies, activation volumes, and their thermodynamic derivatives is developed within the framework of fluctuation theory for dynamics. The method provides the analytical version of an Arrhenius analysis, enabling activation parameters associated with dynamical processes to be obtained from simulations at a single temperature and pressure. Extending a recently developed approach for activation energies [Bartlett et al., J. Chem. Phys. 163, 134114 (2025)], we derive relationships connecting dynamics in constant-energy and volume (and hence constant-enthalpy) ensembles with those in constant-temperature and pressure simulations. Using the diffusion coefficient as an example of a representative dynamical timescale, we show that, for neat water and water in an aqueous dimethyl sulfoxide solution, activation energies and activation volumes can be efficiently computed. In addition, this approach enables direct access to non-Arrhenius behavior through determination of the temperature and pressure dependence of the activation energy and volume.