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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 2: Modeling and Simulation of Soft Matter I

CPP 2.11: Vortrag

Montag, 27. März 2023, 12:30–12:45, MER 02

Pair-Reaction Dynamics in Water: Competition of Memory, Potential-Shape and Inertial Effects — •Florian Brünig, Jan Daldrop, and Roland Netz — Freie Universität Berlin, Department of Physics, 14195 Berlin, Germany

When described by a one-dimensional reaction coordinate, pair-reaction rates in a solvent depend, in addition to the potential barrier height and the friction coefficient, on the potential shape, the effective mass and the friction relaxation spectrum, but a rate theory that accurately accounts for all these effects does not exist. We show how to extract all parameters of the generalized Langevin equation (GLE) and in particular the friction memory function from molecular dynamics (MD) simulations of two prototypical pair reactions in water, the dissociation of NaCl and of two methane molecules. Simulations of the GLE by Markovian embedding techniques accurately reproduce the pair-reaction kinetics from MD simulations without any fitting parameters, which confirms the accuracy of the approximative form of the GLE and of the parameter extraction techniques. By modification of the GLE parameters, we investigate the relative importance of memory, mass and potential-shape effects. Neglect of memory slows down NaCl and methane dissociation by roughly a factor of 2, neglect of mass accelerates reactions by a similar factor, the harmonic approximation of the potential shape gives rise to slight acceleration. This error cancellation explains why Kramers' theory, which neglects memory effects and treats the potential shape harmonically, describes reaction rates better than more sophisticated theories.