The classical equations of motion were integrated using the GROMACS stochastic Langevin algorithm, with a friction constant of 1.0 ps -1 and time step of 2.0 fs -1 . The pressure was maintained using a Parrinello-Rahman pressure coupling constant of 2.0 ps. The Lennard-Jones short-range interactions were smoothly switched off between 12 and 12.5 Å and the electrostatic interactions were computed using the particle mesh Ewald (PME) method with a 12 Å real-space cutoff, 1.0 Å grid spacing, sixth-order spline interpolation, and accuracy of 10 -6 . The free energy of decoupling the solute molecule from its solvent environment was calculated using the GROMACS implementation of the Bennett Acceptance Ratio (BAR) method (gmx bar). We employed a linear decoupling for the electrostatic interaction with six equally spaced λ values (0, 0.2, 0.4, 0.6, 0.8, 1.0) followed by 20 equally spaced λ values (0.0, 0.05, ..., 1.0) with ∆λ = 0.05 to decouple the LJ interactions using the standard GRO-MACS soft-core potential function originally proposed by Beutler et al, with parameters (in GROMACS notation) sc-alpha = 0.5, sc-power = 1 and sc-sigma = 0.3. For each alchemical window, we used a 12.5 ns simulation with the first 2.5 ns discarded for equilibration.