The advantages of using the EVB approach to model chemical reactivity are that wellparameterized force fields can reliably describe thermodynamic properties of interest, while at the same time, as it is a classical approach at its core, this allows for greater conformational sampling than, for example, is possible using DFT-based QM/MM approaches (or approaches using higher levels of theory). In addition, EVB simulations are performed using an energy gap reaction coordinate, which is simply the energy between the two VB states being modelled (Δε = ε1 -ε2, where ε1 and ε2 are the energies of the different VB states, Figure ). While there is a geometric component to these, as the energies of the individual states are based on the geometries of the structures in each of the VB states, the advantage of using the energy gap reaction coordinate, as opposed to a pure geometric reaction coordinate, is that it allows for the full multidimensional configurational space of the reacting system to be projected onto a single energy-based reaction coordinate . This, in turn, accelerates the convergence of the free energy calculations . Despite these advantages, it can be attractive to further improve the sampling of the dynamical properties of the system along the chemical reaction coordinate during EVB simulations, and, indeed, this was successfully done by Liu and Warshel in a study of dihydrofolate reductase (DHFR) , who used a protocol very similar to replica exchange to enhance the sampling of the hydride transfer catalyzed by this enzyme. Specifically, they performed their EVB-FEP/US simulations using 31 mapping windows (λm) of 180 ps in length, exchanging the system at adjacent frames every ps, performing the exchanges between frames (λ1, λ2)(λ3, λ4)…( λ29, λ30) on odd swap cycles, and (λ2, λ3)(λ3, λ4)…(λ30, λ31) on even swap cycles. This is formally similar to averaging the simulations over different initial conditions as well as running both EVB trajectories both backwards and forward between the different VB states, but allowed them to achieve this using a single 5.6 ns trajectory (31 x 180 ps) run in parallel, thus improving the sampling while simultaneously reducing the simulation time. Inspired by this, we have developed an open-source Python-based tool Q-RepEx, that interfaces with the Q6 simulation package , allowing for EVB-FEP/US simulations to be performed within a replica-exchange framework (REMD-EVB).