This paper describes fragment molecular orbital (FMO) calculations for nine different complexes of HIV-1 protease and its inhibitors. All FMO calculations were carried out at the FMO2-MP2/6-31G* level by using the MIZUHO/ABINIT-MP program on the K computer and on the TSUBAME computer of Tokyo Institute of Technology. We computed the inter-fragment interaction energies (IFIEs) between the inhibitor and the amino acid residues in HIV-1 protease and estimated sum of IFIEs as binding energy between an inhibitor and HIV-1 protease. FMO computations show that the binding energy can be well correlated to the dissociation constant, Kd value, of each inhibitor. Our results suggest that it is certainly possible to evaluate and to predict the activity values of individual inhibitors.
