Kinetically constrained ring-polymer molecular dynamics extended to all regimes of electronic coupling.

Kinetically constrained ring-polymer molecular dynamics (KC-RPMD) is an approximate quantum dynamical method for treating electronically non-adiabatic processes in condensed-phase systems, predicting accurate electron transfer (ET) rate constants in the normal and inverted Marcus regimes through computationally efficient classical equations of motion. The favorable properties of KC-RPMD in the weak diabatic coupling regime have been well investigated. However, an in-depth study of its performance in the strong coupling regime has not been performed until now. In this work, we identify a breakdown of free energies and ET rates predicted by traditional KC-RPMD in the strong coupling regime. We extend KC-RPMD to allow for the direct simulation of systems spanning the full range of diabatic coupling strengths, maintaining all the favorable properties in the weak coupling regime while recovering the appropriate adiabatic description in the strong coupling regime. This is accomplished through a modification to the Gaussian restraint penalty function, which is made to gradually loosen and vanish at strong coupling. We demonstrate the accuracy of KC-RPMD on a variety of spin-boson model systems spanning a wide range of diabatic coupling strengths.