In perturbation simulations, lambda steps between 0.0, 0.02, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8,0.9, 0.95, 0.98, 1.0 were used, with 3 ns of simulation used for each lambda value. To prevent the protein from drifting off the membrane surface due to low natural affinity, harmonic constraints were applied to membrane phosphates as well as the alpha carbons of h-cisPT during the FEP simulation so that the bound state is preserved, even if the interaction is unfavorable. The potential energy function of the harmonic restraint is