Kinetically constrained ring-polymer molecular dynamics for non-adiabatic chemical reactions.

We extend ring-polymer molecular dynamics (RPMD) to allow for the direct simulation of general, electronically non-adiabatic chemical processes. The kinetically constrained (KC) RPMD method uses the imaginary-time path-integral representation in the set of nuclear coordinates and electronic states to provide continuous equations of motion that describe the quantized, electronically non-adiabatic dynamics of the system. KC-RPMD preserves the favorable properties of the usual RPMD formulation in the position representation, including rigorous detailed balance, time-reversal symmetry, and invariance of reaction rate calculations to the choice of dividing surface.

Long Range Proton-Coupled Electron Transfer Reactions of Bis(imidazole) Iron Tetraphenylporphyrins Linked to Benzoates.

Concerted proton-electron transfer (CPET) reactions in iron carboxy-tetraphenylporphyrin complexes have been investigated using both experimental and theoretical methods. Synthetic heme models abstract H and e from the hydroxylamine TEMPOH or an ascorbate derivative, and the kinetics of the TEMPOH reaction indicate concerted transfer of H and e. Phenylene linker domains vary the electron donor/acceptor separation by approximately 4 Å.

Direct simulation of electron transfer using ring polymer molecular dynamics: comparison with semiclassical instanton theory and exact quantum methods.

The use of ring polymer molecular dynamics (RPMD) for the direct simulation of electron transfer (ET) reaction dynamics is analyzed in the context of Marcus theory, semiclassical instanton theory, and exact quantum dynamics approaches. For both fully atomistic and system-bath representations of condensed-phase ET, we demonstrate that RPMD accurately predicts both ET reaction rates and mechanisms throughout the normal and activationless regimes of the thermodynamic driving force. Analysis of the ensemble of reactive RPMD trajectories reveals the solvent reorganization mechanism for ET that is anticipated in the Marcus rate theory, and the accuracy of the RPMD rate calculation is understood in terms of its exact description of statistical fluctuations and its formal connection to semiclassical instanton theory for deep-tunneling processes.

Ring polymer molecular dynamics beyond the linear response regime: Excess electron injection and trapping in liquids.

Ring polymer molecular dynamics (RPMD) is used to directly simulate the injection and relaxation of excess electrons into supercritical helium fluid and ambient liquid water. A method for modulating the initial energy of the excess electron in the RPMD model is presented and used to study both low-energy (cold) and high-energy (hot) electron injections. For cold injection into both solvents, the RPMD model recovers electronically adiabatic dynamics with the excess electron in its ground state, whereas for hot electron injection, the model predicts slower relaxation dynamics associated with electronic transitions between solvent cavities.