Designed inhibitors of 3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase from previous experiments show high binding affinities. In this work, the binding affinities of these designed inhibitors are determined via free energy simulations. The calculated values agree well with the corresponding experimental results. Several structurally different inhibitors share extremely similar binding affinities. Detailed interaction analysis including the hydrogen bond profiles and the interaction energies between the ligand and its surroundings shows large enthalpy changes upon mutation from one ligand to another, while the binding affinities remain unchanged. This finding indicates the existence of entropy-enthalpy compensation.