Electron transfer (ET) plays a central role in a wide range of chemical and biological processes involving redox, energy transfer, and catalysis. Consequently, many computational techniques for elucidating the mechanism and timescales of ET in extended systems have been proposed. One such method is called Kinetically constrained ring-polymer molecular dynamics (KC-RPMD), an approximate quantum dynamical method for treating electronically non-adiabatic ET in condensed phase systems through computationally efficient classical equations of motion. KC-RPMD is unique in its ability to sample rare event configurations governing the ET mechanism. For example, this enabled the elucidation of the fluctuating hydrogen-bond network which governs electron transfer in the blue copper protein azurin. The performance of KC-RPMD for electronically non-adiabatic processes has been well-investigated, however, the methodology had not been applied to systems exhibiting electronically adiabatic ET until now.

In this work, we identify a breakdown of traditional KC-RPMD for electronically adiabatic processes characterized by strong diabatic coupling. We extend KC-RPMD to allow for the direct simulation of systems exhibiting both electronically adiabatic and non-adiabatic ET. This new extension of KC-RPMD maintains all the favorable properties at weak diabatic coupling while simultaneously recovering the appropriate adiabatic description for strong diabatic coupling. We demonstrate the accuracy of this new KC-RPMD implementation on a range of spin-boson model systems dynamically spanning electronically adiabatic and non-adiabatic regimes of ET. The result is a robust and efficient computational methodology which enables rate calculations and elucidates the underlying ET mechanisms for arbitrary diabatic coupling in biologically relevant extended systems.

Joshua Kretchmer

Georgia Institute of Technology

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