To obtain more detailed information about the affinity of paraoxon towards WT OPH and OPH mutant forms, we performed absolute binding free energy calculations in implicit solvent on these different complexes. Although binding free energy calculations in implicit solvent may not be as accurate as those in explicit solvent, we wanted to test the accuracy of this approach on our studied protein target and to assess the potential strength of OPH and OPH-mutants interactions with paraoxon within the limitations of the method.