Andersen thermostat (collision frequency 10 ps -1 ) and Monte Carlo barostat (25 time steps between isotropic box scaling attempts) were used to maintain a temperature and pressure of 298 K and 1 atm. A timestep of 4 fs was used in combination with the leap-frog Verlet integrator and hydrogen mass repartitioning (using a repartitioning factor of 4). All bond lengths were constrained. The reaction field method was used with a dielectric constant of 78.3, 66 and a cut-off of 12 Å was used for all non-bonded interactions. Energy minimisation was performed prior to each simulation with a maximum of 1000 iterations. The bound stage vanish λ windows were run for 8 ns, and all others for 6 ns. Free energy differences for each stage were estimated using the Multistate Bennett-Acceptance Ratio (MBAR) for the final 5 ns of all simulations. Coordinates were saved every 20 ps.