From the generated trajectories the first 2.25 ns were discarded and the remaining simulation frames were used to initialise alchemical non-equilibrium transitions between the two end states: 80 transitions in each of the two directions. 50 ps long non-equilibrium alchemical transitions were started from each frame and the work needed to perform the transition was recorded. Relative binding free energies were calculated from the bi-directional work distributions using a maximum-likelihood estimator implemented in pmx . The equilibrationtransitions-analysis protocol was repeated five times for each evaluated ligand and the mean and standard error of the five repeats were taken as the relative binding free energy and associated uncertainty. To obtain the absolute binding free energy for use in the training set, the relative free energy was combined with the experimentally known absolute binding free energy of the reference ligand.